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Resource Management Technical Reports
2008
Understanding soil analysis data
M A. Hamza
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Understanding
soil analysis
data
RESOURCE MANAGEMENT
TECHNICAL REPORT 327
ISSN 1039-7205
Resource Management Technical Report 327
Understanding soil analysis data
MA Hamza
July 2008
Disclaimer:
The contents of this report were based on the best available information at the time of
publication. Conditions may change over time and conclusions should be interpreted in the
light of the latest information available.
© Western Australian Agriculture Authority, 2008
UNDERSTANDING SOIL ANALYSIS
Acknowledgments
The cost of the first edition of this publication has been wholly met from the South Coast Soil
Health Initiative project managed by Tim Overheu (core funding also provided by the
Western Australian and Australian Governments through the National Action Plan for Salinity
and Water Quality, hosted by South Coast Natural Resource Management Inc.). I would also
like to thank Tim for his helpful comments, contributions, valuable suggestions and
coordination support to finalise this publication.
Additional thanks go to Wal Anderson, Ross Brennan, Mike Bolland, Chris Gazey and Ruhi
Ferdowsian from the Department of Agriculture and Food, Western Australia, and to Dave
Allen from the Chemistry Centre, Western Australia for their technical comments and
suggestions on draft versions.
Written permission to reproduce some of the images throughout this document has also
been granted by ‘Pearson Education’.
Front cover
The front cover shows an aerial view of the North Stirling basin with a Department of
Agriculture and Food trial site in the foreground.
2
UNDERSTANDING SOIL ANALYSIS
CONTENTS
Page
INTRODUCTION .................................................................................................................
5
1.
FORMS OF NUTRIENTS IN SOILS ................................................................................
7
2.
SOIL PHYSICAL PROPERTIES .................................................................................... 10
3.
CALCIUM TO MAGNESIUM (CA: MG) AND OTHER CATION RATIOS ................................ 20
4.
SOIL REACTION, PH ................................................................................................. 22
5.
SODICITY ................................................................................................................ 29
6.
SALINITY (EC) ......................................................................................................... 30
7
CATION EXCHANGE CAPACITY (CEC) ....................................................................... 32
8.
ORGANIC MATTER (OM) .......................................................................................... 34
9.
NITROGEN (N) ......................................................................................................... 37
10.
SULPHUR (S) ........................................................................................................... 38
11.
PHOSPHORUS (P) .................................................................................................... 39
12.
CALCIUM (CA) ......................................................................................................... 41
13.
MAGNESIUM (MG) .................................................................................................... 41
14.
POTASSIUM (K) ....................................................................................................... 43
15.
COBALT (CO) .......................................................................................................... 44
16.
BORON (B) .............................................................................................................. 44
17.
IRON (FE) ................................................................................................................ 46
18.
MANGANESE (MN) ................................................................................................... 46
19.
COPPER (CU) .......................................................................................................... 47
20.
ZINC (ZN) ................................................................................................................ 48
21.
MOLYBDENUM (MO) ................................................................................................. 48
22.
SODIUM (NA) ........................................................................................................... 49
23.
ALUMINIUM (AL) ....................................................................................................... 50
Glossary..............................................................................................................................50
REFERENCES .................................................................................................................... 54
3
UNDERSTANDING SOIL ANALYSIS
4
UNDERSTANDING SOIL ANALYSIS
Introduction
The aim of this report is to help people who are interested in soil science, but are not
specialists in this area, to better understand soil analysis reports in particular, and soil
data in general.
An important starting point is acknowledging that agricultural laboratories express analytical
results differently. Some are easier to understand than others, but many recipients see only
a long list of incomprehensible figures. This publication contains information to assist with
the understanding of results from common agricultural analyses. It is not intended to make
recommendations for application of fertilisers, or to suggest that one form of analytical data is
more useful than any other. It is important however, that any interpretations are cross
checked with advice from a local agronomist. It is also important to acknowledge that
specialist analyses may not be covered in this document.
When it comes to selecting the right set of tests, your local Department of Agriculture and
Food soil resource extension officer or agronomist can help determine the best analytical
tests for your farm. When choosing a laboratory to submit your soil samples, you need to
make sure it has quality accreditation. This publication has been written for Western
Australia. However, the principles apply in all areas. Check with your local agronomist to
determine any differences in preferred levels for your region.
Soil analysis reports are generally tailored to provide the required information for both
physical and chemical fertilities of the soil. Some soil reports may also include biological
analysis. Soil physical fertility represents the ability of the soil to store and conduct water,
nutrients and gases. Whereas soil chemical fertility indicates whether there are enough
available nutrients for plant growth, or whether fertilisers are needed to correct deficiencies.
Soil physical fertility information should include soil texture while the chemical fertility
information should cover cation exchange capacity (CEC), the percentage of exchangeable
cations, pH, electrical conductivity (EC), organic carbon (matter) and the concentrations of
essential macro and micronutrients in the soil, as well as some other elements such as
aluminium. Lime, gypsum or dolomite requirements are also included. Organic matter in the
soil is important for both chemical and physical fertility and should be an important part of
any soil report.
The chemical tests should address accurately both nutritional and toxicity considerations.
Ideally, the soil test should not only identify whether a nutrient deficiency exists, but also the
degree of deficiency in terms of expected yield loss. It is important that the quantity of
nutrient required to correct the deficiency is then determined by field calibration trials. An
ideal soil test is one that is reproducible and rapid, as well as being reliably correlated with
local responses in plant yield or nutrient uptake.
Calibrating soil test in the field
Two calibration approaches are common in soil fertility:
x
Comparing fertilised and unfertilised plots: to quantify plant yield response, the relative
yield in the presence (Yf) and absence (Ynf) of the applied nutrient(s) are examined.
Relative yield (%) = 100 (Yuf /Yf)
Relative yield response (%) = 100 [(Yf – Ynf) /Yf]
x
The examined nutrient(s) are applied as treatments to selected soils of varying soil
nutrient status. Yields are then compared statistically for significant increase.
5
UNDERSTANDING SOIL ANALYSIS
These two approaches show if there are adequate or deficient levels of a particular
nutrient(s) in the soil and do not take into account the effect of other soil or environmental
constraints which might affect yield.
Most of the soil samples must be dried and ground before analysis. To determine the soil
fertility level, nutrients must be removed from the soil exchange sites (clay minerals and
organic matter) to the soil solution where they can then be extracted and measured.
Principle of soil extraction
Most of the soil chemical tests are based on extracting elements from the soil solid phase to
a soil-free liquid phase (extractant) and then measuring the concentration of these elements
in the soil-free liquid phase. The extraction can be done by one or more of the following
processes:
x
Ion exchange/desorption: Where the elements are replaced from the soil solid phase
by an element that is more strongly adsorbed to the soil exchange sites. Plant
available phosphorus (P) and sulphur (S) in Australian soils are commonly extracted by
this method. This method also forms the basis for some of the new techniques to
extract nutrients from soil by applying cation/anion exchange resin strip or capsules,
and filter paper strips saturated with iron oxide.
x
Dissolution: Dissolving the elements by water or a more vigorous extracting solution
such as an acid or ligand (a molecule, atom or ion bonded to the central metal atom or
ion in a coordination compound). In Australia, water is used to extract soluble salts
from soil and for measurement of electrical conductivity, gypsum and chloride
(Rayment & Higginson 1992b). Acids such as hydrochloric and sulphuric are
commonly used to dissolve sparingly soluble calcium phosphate in soil.
x
Complexation: The availability to plants of micronutrients such as zinc (Zn), iron (Fe),
copper (Cu), manganese (Mn) and P is limited by the rate of diffusion through soil,
particularly in neutral and alkaline soils. Complexation of these ions in the rhizosphere
then becomes an important step in accessing them by plant roots (Lindsay 1974).
Accordingly, complexing agents have been used widely in testing for micronutrients in
soil. The complexation is achieved when an element in the soil is complexed or paired
with a complexing ion or compound, changing the effective charge on the element and
retaining it in solution in a complexed form for subsequent analysis.
x
Oxidation/reduction: Some elements undergo a distinct change in chemical
behaviour as a result of either reduction or oxidation. These changes cause the
element to be released from the solid phase into either solution or gaseous phase for
analysis. An example of this method is the determination of soil organic carbon.
Soil depth
Plant roots are commonly more extensive in the subsoil. Accordingly the subsoil
environment is more relevant to root growth than the topsoil. However, most fertilisers and
soil amendments are applied to the topsoil rather than into the subsoil for practical reasons,
which can be problematic. For example, if lime or immobile nutrients were applied to the
topsoil, they would not move down in an appreciable amount to meet crop demands in time.
Topsoil is important in other aspects. In rain-fed areas, it separates the subsoil from the
atmosphere where the main source of moisture is. It also contains the main accumulation of
organic matter and applied fertilisers. Organic matter represents the main residual source of
nitrogen (N) in the soil and N content of organic matter must be known before estimating how
much N fertiliser needs to be applied. Topsoil is usually high in P because only part of P
fertiliser is recovered every year and most P fertilisers are relatively insoluble.
6
UNDERSTANDING SOIL ANALYSIS
Plant nutrient movement within the soil profile is a complex issue because it is affected by
many soil parameters such as pH. It is hard to establish a clear cut line defining borders
between different nutrient concentrations at different soil depths. In general, for a
satisfactory soil test, both topsoil (0-10 cm) and subsoil (10-40 cm) should be sampled.
Some agricultural laboratories provide soil sample kits which can be helpful in soil sampling.
The kits can instruct how to collect a number of soil samples (usually topsoil) which are
mixed together in a bag. This means that the analytical report will be an average of the
combined samples. However, averages can hide problems if there is a big range in the soil
sampled. Areas where vegetation changes or where plant vigour is different are likely to
have different soil conditions and these areas should be tested separately.
1. Forms of nutrients in soils
In soil, most plant nutrients are present as minerals, organic matter, exchangeable ions or in
solution. The bulk of these nutrients are not readily available for the plant. They only
become available through chemical, physical and biological processes which are mostly not
fast enough to meet plant demands. In general, the short term supply of cations to plants is
governed by the established equilibrium between exchangeable cations and their
concentrations in the soil solution. Table 1 shows the average of how many nutrients are
contained per tonne of grain of different crops.
Table 1.
Average nutrients contained in 1 tonne of grain of different crops. Note that
amounts can vary by as much as 30 per cent due to differences in soil fertility level,
crop varieties and seasons (published and unpublished data)
Crop
N
P
K
Ca
Mg
S
Cu
Zn
Mn
B
Mo
kg/t
kg/t
kg/t
kg/t
kg/t
kg/t
g/t
g/t
g/t
g/t
g/t
Barley
21
3
4
0.4
1.5
2
5
17
15
2
0.3
Wheat
23
3
4
0.3
1.3
2
4.5
22
35
4.5
0.5
Chickpea
33
3.2
9
1.6
1.4
2
7
34
34
< 0.1
< 0.1
Faba beans
41
4
10
1.3
1.2
1.5
10
28
14
13
3
Field peas
38
3.4
9
1
1.3
1.8
5
35
14
-
-
Canola*
35
6
9
4
4
9
6
40
30
16
0.3
Lupins
55
3.5
10
2.7
2
2.4
5
30
60
20
4
Oats
20
3
4
0.5
1.5
2
5
25
37
5
Hay
30
3
25
-
-
2
-
-
-
-
0.5
-
When interpreting a soil report we should consider that grain yield will be determined by the
least available essential nutrient, whether this is a macro or micronutrient (Figure 1). This
concept is called the ‘Law of the minimum’ and was introduced by the German chemist,
Justus von Liebig. It indicates that plant growth is constrained by the essential element that
is most limiting. Other limitations could be moisture or any chemical or physical factors. If a
factor is not limiting, increasing it will do little or nothing to enhance plant growth. In fact,
increasing it may actually reduce plant growth by throwing the system further out of balance.
In general, there are 16 essential elements for plant growth. An essential element for plant
growth can be defined as follows:
1.
It cannot be substituted by any other element.
2.
The plant can not complete its life cycle without it.
7
UNDERSTANDING SOIL ANALYSIS
3.
It is directly involved in the nutrition of the plant as a constituent of an essential
metabolite or required for the action of an enzyme system (Mengel & Kirkby 1982).
Yield potential (%)
100
80
60
40
20
0
N
P
K
Ca
Mg
S
Cu
Zn
Mn
B
Mo
Nutrients
Figure 1.
An illustration of the ‘Law of the minimum’ where copper (Cu) is the least available element,
resulting in only 60 per cent of the potential yield.
The 16 essential elements or nutrients are:
Carbon
C
Magnesium
Mg
Hydrogen
H
Iron
Fe
Oxygen
O
Manganese
Mn
Nitrogen
N
Copper
Cu
Phosphorus
P
Zinc
Zn
Sulphur
S
Molybdenum
Mo
Potassium
K
Boron
B
Calcium
Ca
Chlorine
Cl
Oxygen and carbon are obtained from the air, while hydrogen is obtained from splitting the
water inside the plant. All the other 13 elements are absorbed from the soil. Silicon and
sodium might be required by some higher plants and cobalt may be essential for some
legumes (Chatel et al. 1978). Nutrients interact with each other, resulting in either a
decrease (antagonism) or an increase (stimulation) in their availability to the plant.
In general, plant nutrients can be classified into four groups depending on their biological
functions (Table 2, Mengel & Kirkby 1982).
8
UNDERSTANDING SOIL ANALYSIS
Table 2.
Classification of plant nutrients
Nutrient element
Most common uptake form
-
C, H, O, N, S
CO2, HCO3 , H2O, O2,
+
2NO3 , NH4 , N2, SO4 ,
SO2
P, B
H2PO4-, HPO42-,, H3BO3, H2BO3-
+
2+
+2
2+
Organic matter constituents. Involved in
enzymic processes. Assimilation by
oxidation-reduction reaction.
-
K, Mg, Ca, Mn, Cl
K , Mg , Ca , Mn , Cl
Fe, Cu, Zn, Mo
Fe , Cu , Zn , MoO4
2+
2+
2+
Biological function
Esterification with native alcohol groups in
plants. P-esters are involved in the energy
transfer reaction (TPA to DPA).
Establishing osmotic potential, enzyme
activation, balancing anions. Maintaining
membrane permeability and electron
potential.
Enable electron transfer by valency change.
Present mainly in chelated* form incorporated
in prosthetic** groups.
2-
*
A molecular structure in which a central polyvalent metal ion is combined into one or more rings by a chelating
agent.
** A non-protein molecule which is combined with a protein, such as the haem group in haemoglobin.
Besides Table 2, which classifies nutrients into groups, some important functions of the
micronutrients are:
x
Zinc, Zn: Promotes growth hormones and starch formation, promotes seed maturation
and production
x
Iron, Fe: Important in chlorophyll formation, N fixation, NO3 and SO42 reduction
x
Copper, Cu: Important in photosynthesis, protein and carbohydrate metabolism, and
probably N fixation
x
Magnesium, Mg: Important in photosynthesis, N metabolism, and N assimilation
x
Boron, B: Facilitates sugar translocation and synthesis of nucleic acids and plant
hormones, essential for cell division and development
x
Molybdenum, Mo: Essential for nitrogen fixation and assimilation because it is present
in nitrogenase (nitrogen fixation) and nitrate reductase enzymes
x
Cobalt, Co: Essential for nodule function and nitrogen fixation in legumes
x
Chloride, Cl: Essential in photosynthesis, regulates water uptake in salt-affected soils.
It is clear from the functions of the nutrients that N, which is taken up by most higher plants in
quantities more than any other essential element, would not be utilised by the plant without
the presence of Mo which may be the element absorbed least. This emphasises the fact that
all essential elements have the same importance for the plant. Figure 2 shows the average
level of micronutrients in plants.
9
UNDERSTANDING SOIL ANALYSIS
0
Concentration in p ant, ppm, og sca e
0.1
Deficiency
1
Normal
Toxicity
10
100
1000
Mo Cu
Figure 2.
B
Zn
Mn Fe
Level of micronutrients in plants in mg/kg or ppm (based on data
from many sources, reconstructed from Brady & Weil 2002).
2.
Soil physical properties
2.1
Soil texture
Soil texture is not readily subject to change, so it is considered a basic property of a soil. Soil
texture refers to the size (diameter) of individual soil particles. There are three broad groups
of textural classes: sand, loam and clay (Figure 3). The size differences between these
classes give rise to significant differences in other physical properties such as pore size and
cation exchange capacity, which play a vital role in storing and transporting water, gases and
nutrients in soil (see Table 3).
Table 3.
Some physical properties of the major soil texture classes
Texture class
Size (mm)
Number of particles
per gram
Coarse sand
2.0-0.2
5 x 102
20
5 x 10
5
200
5 x 10
8
2,000
5 x 10
11
Fine sand
Silt
Clay
0.2-0.02
0.02-0.002
< 0.002
10
Specific surface area
(cm2/g)
20,000 (2 m2)
UNDERSTANDING SOIL ANALYSIS
Figure 3.
Major soil textural classes are defined by the percentages of sand, silt and clay (after Marshall
1947).
Table 3 shows clearly that the most important classes in soil texture are clay and to lesser
extent silt, since they possess hundreds of times more surface area (hence more
electrostatic adsorption capability) than sand. This enables the soil to store many more
nutrients and also to buffer the harmful effect of sudden chemical changes in the soil. Most
natural agricultural soils contain some clay. Even sandy soils at the lowest left hand corner
of the triangle (Figure 3) may contain up to 10 per cent clay. This small fraction of clay is
what makes such soils suitable for broadacre agriculture. For example, one gram of
extremely sandy soil (95% sand + 5% clay or a ratio of 19:1 sand to clay) contains only
19 cm2 surface area of sand but 1,000 cm2 surface area of clay. The ratio for sand:clay
surface area is 1:53.
Another important function of clay is to act as a binding agent, helping it to aggregate and
maintain good soil structure (Figure 4).
11
UNDERSTANDING SOIL ANALYSIS
Figure 4.
Polarised light microscopic image for a thin section of a loamy soil. Sand and silt particles
shown are irregular in size and shape, the silt being smaller. Clay film can be seen coating
the wall of the large pores (arrow) and binding other soil particles. Empty pores appear black
(courtesy of M. Rabenhorst, University of Maryland).
Furthermore, clay and to a lesser extent silt, help to maintain bio-pores in soil by coating
them, thus providing a network of conducting channels (Figure 5).
Figure 5.
Conducting vessels coated by clay and silt.
Soil texture is determined by establishing how much sand, silt and clay are in a soil sample.
This is commonly done by a modified ‘plummet’ procedure. Soil is dispersed with a solution
of ‘Calgon’ (sodium hydroxide), then silt (0.020-0.002 mm) and clay (<0.002 mm) are
measured by density measurements using a plummet after standard settling times (Loveday
1974).
2.2. Soil structure
Soil structure refers to soil aggregation and is defined as the combination or arrangement of
primary soil particles (sand, silt and clay) into secondary units or peds. The secondary units
are characterised on the basis of size, shape and grade. Soil structure is the main soil
physical fertility parameter that affects water and nutrient transport and storage in the soil.
There is a direct relationship between the dominant clay type in soil and the ability of the soil
to form and maintain its structure. A brief discussion of clay minerals in soil may be helpful
for understanding soil texture classes.
12
UNDERSTANDING SOIL ANALYSIS
Clay minerals
Most clay minerals in soil are silicates. They carry negative charges which play a vital role in
soil structure and plant nutrition. However, there are some non-silicate clays which do not
carry electrostatic charges, such as iron and aluminium oxides and hydroxide clay, which is
common in highly weathered soils like the ones in Western Australia (Gibbsite, Al(OH)3 is a
common non-silicate clay). Most silicate clay minerals are composed of microscopic sheets
in a flake shape or hexagonal crystals (Figure 6). There are two dominant types of clay
which influence soil structure: 1:1 clay which is common in the highly weathered soils of WA,
and 2:1 clay which dominates less weathered soils in the rest of the world (see Figure 6).
P
0.5Pm
Kaolinite , 1:1
Figure 6.
Two silicate clay minerals, kaolinite, 1:1 clay (left) and mica (illite), 2:1 clay (right)
(Courtesy of Dr Bruce F. Bohor, Illinois State Geological Survey, Soil Science Society of America)
The basic molecular and structural components of silicate clays are:
x
Tetrahedron: Four-sided building block composed of a silicon ion surrounded by four
oxygen atoms
x
Octahedron: Eight-sided building block in which Al or Mg ions are surrounded by six
hydrogen groups or oxygen atoms (Figure 7).
The classification of clay minerals into the two main silicate groups 1:1 and 2:1 is based on
the number and arrangement of tetrahedral (Si) and octahedral (Al, Mg) sheets contained in
the crystal units or layer. In 1:1 silicate clay, each layer contains one tetrahedral and one
octahedral sheet. In 2:1 silicate clays each layer has one octahedral sheet sandwiched
between two tetrahedral sheets (Figure 8).
13
UNDERSTANDING SOIL ANALYSIS
Al
Mg
O
Si
OH
H
Tetrahedron
Octahedron
Octahedral sheet
Layer
Tetrahedral sheet
Adsorbed can ions and water
Interlayer
Tetrahedral sheet
Octahedral sheet
clay crystal
Tetrahedral sheet
Layer
Tetrahedral sheet
Figure 7.
A single tetrahedron and octahedron, the two major building blocks for silicate clay mineral.
In the clay crystal thousands of tetrahedral and octahedral building blocks are connected to
give planes of Si and Al (or Mg) ions.
Tetrahedron
Octahedron
Hydroxide sheet
1.4 nm
0.7 nm
1- 2 nm
Kaolinite (1:1)
Nonexpanding clay
(no swelling)
Water and
cations
Chlorite (2:1)
Nonexpanding clay
(min. swelling)
Smectite (2:1)
Expanding clay
(max. swelling)
Figure 8.
Schematic drawing illustrating 1:1 and 2:1 clay minerals.
14
UNDERSTANDING SOIL ANALYSIS
Note that 1:1 clay, such as kaolinite is not an expanding clay because its layers are held
together by hydrogen ions. Maximum interlayer expansion is found in smectite (Figure 9)
and vermiculite (with somewhat less expansion) because of the moderate binding power of
numerous Mg2+ ions. Expansion occurs when water enters the interlayer spaces forcing the
layers apart. Some 2:1 clay minerals, such as fine-grained illite and chlorite do not expand
because K+ ions (for example in case of fine-grained illite) or an octahedral-like sheet of
hydroxide of Al, Mg, and Fe tightly bind the 2:1 layers together.
Figure 9.
2:1 expanding clay mineral smectite. This clay mineral has a maximum swelling power
(Courtesy of USDA Natural Resources Conservation Service)
Source of negative charges in silicate clay minerals
During the weathering of rocks and minerals, many different elements coexist together in the
weathered solution. As the clay minerals are crystallised, cations of comparable size may
substitute for Si, Al and Mg ions in the tetrahedral or octahedral sheet causing unbalanced
charges to occur in the clay lattice (Figure 10).
3+
3+
23+
2-
2-
2-
3+
2-
2-
2-
2-
6+
3+
2+
6-
2-
2-
6-
2-
65+
2-
63+
3+
Net = -1
Net = 0
Di-octahedral
With isomorphic substitution
Di-octahedral
2-
Oxygen
H
2+
Mg or Fe
3+
Al
Figure 10. Isomorphic substitution, here a magnesium or iron ions with +2 charges replacing an
aluminium ion with 3+ charge causing the net charge to be -1.
15
UNDERSTANDING SOIL ANALYSIS
Soil compaction
Soil compaction is a major deterioration in soil structure. Even though large areas of
Western Australia suffer from soil compaction, many farmers are not aware of the problem
because it occurs in the subsoil and is seldom visible at the surface (Figure 11).
Figure 11. Soil erosion exposes soil compaction
(image courtesy of Chris Gazey, DAFWA).
Compacted soils usually have massive soil structure (sometimes called structureless) and
high bulk density (Figure 12). In well structured soils, the pore spaces are relatively large
and connected (Figure 13). Whereas in compacted soils, the pores are usually crushed and
hydraulic continuity is disrupted. Compacted soils usually store much less soil solution and
transport it more slowly than soils with good structure. In addition, roots need to exert more
energy to penetrate the soil, energy which otherwise would be allocated for plant growth and
yield. The limited amount of energy available leads to small, shallow root systems
(Figure 14) and makes it difficult for seedlings to emerge (Figure 15). When soil strength
exceeds 2.5 MegaPascals (MPa) most plant roots cease growing (Figure 16).
Figure 12. Compacted, massive clods brought up to the soil surface upon ripping a sandy soil in
Tammin, Western Australia.
16
UNDERSTANDING SOIL ANALYSIS
Figure 13. A three dimensional scan image of the network of pores in undisturbed soil (edges about
2 mm long). The pores (light in colour) show great variability in size and cross-sectional area.
Not all pore channels are connected to each other leading to isolation of some tiny pockets of
air and water, hence preventing them from moving readily downward or upward in the soil
(Courtesy of Dr Isabelle Cousin, INRA Unite de Science du sol, SESCPF Centre de Research
d’Orleans de Limere, Ardon, France)
A
Figure 14.
B
Left: Corn crop with fibrous roots growing in compacted and uncompacted subsoils.
Right: Black-berried nightshade weeds with tap roots growing in compacted and
uncompacted subsoil (image B courtesy of Tim Overheu). Note the horizontal short growth of
the primary tap roots in compacted soil compared to the vertical more extensive root growth
in the uncompacted subsoil.
17
UNDERSTANDING SOIL ANALYSIS
0
0.5
Moderate decrease in root growth
Moderately Compacted soils
Plant seedlings must force their way through the soil surface as in this picture. In
compacted soils this process is more difficult (Courtesy of R Weil).
No decrease in root growth
root growth
Un-compacted soils
Figure 15.
compacted soils
Growth almost
ceases
Significant decrease
in root growth
1.0
1.5
2.0
2.5
Soil strength, MPa
Figure 16. Relationship between root growth and soil strength.
The presence of specific cations in the soil helps in forming and maintaining good soil
structure because they help soil aggregation. To understand the role of cations in soil
structure it is important to discuss how soil structure is formed.
How soil structure is formed
Soil structure depends on the ability of clay particles (flakes or crystals) to flocculate (form
aggregated or compound masses of particles). Flocculation is a balance between two
opposing forces which act on the clay particles: a repulsive force which prevents flocculation
and an attractive force which causes flocculation. The repulsive force occurs between
negatively charged clay particles, while the attractive force is Van Der Waal’s force.
Van Der Waal’s force is a weak attractive force between non-polar, electrically neutral
molecules or parts of molecules when they lie close together. It acts over a very short range
and is inversely proportional to the seventh power of the distance between the atoms or
molecules. This microscopic distance coupled with the weakness of the force makes it easy
for the repulsive force to be dominant, preventing flocculation and causing soil dispersion.
To overcome this dominance, a large proportion of the negative charges on the clay particle
must be neutralised to reduce the repulsive force and allow the attractive force to be active.
This can be achieved by adding any cation (e.g. calcium) to the soil (Figure 17). The higher
the cation valence (number of charges per ion) the higher the neutralising value of that ion.
18
UNDERSTANDING SOIL ANALYSIS
Figure 17. Calcium ions help clay particles to flocculate by neutralising negative charges, thus reducing
the repulsive forces between them.
For example, flocculation can be produced in the soil by addition of Al, Ca or Na in the
relative amounts of approximately 0.04 Al3+, 2Ca2+ or 100 Na+ (Taylor & Ashcroft 1972).
However, at these relative amounts both Na and Al will be harmful for both soil and plant, so
Ca is the best practical choice to produce flocculated soil. It can be applied as gypsum
(CaSO4.2H2O) and lime (CaCO3) to address mild soil compaction. However, severe soil
compaction must be removed by physical means, such as ripping after applying calcium.
Ripping soil in the absence of an aggregating agent such as gypsum or organic material
benefits the soil only for a short period, after which the soil becomes compacted again.
Crops with strong, deep taproots could also be used to deal with soil compaction. Roots
shrink (on transpiration) and swell (at night when transpiration has ceased) pushing and
disturbing soil around them, thus helping to mitigate compaction (Hamza et al. 2001,
Figure 18).
100 P
Root
Soil
Figure 18. Scanning electron micrograph of a cross-section of a peanut root surrounded by soil. Plant
roots shrink and swell diurnally, destabilising soil around them and loosening compaction.
19
UNDERSTANDING SOIL ANALYSIS
3.
Calcium to magnesium and other cation ratios
While the role of exchangeable Ca in maintaining soil structure especially in medium to
heavy soils is well known and documented, it has proven difficult to establish critical levels of
exchangeable Ca for plant growth that apply across a range of dissimilar soils (Bruce 1999).
Pierre (1931) was one of the first to emphasise the importance of exchangeable Ca in acid
soils and to suggest that base saturation (see Glossary for definition) was more important
than the absolute amount of exchangeable Ca. He also concluded that the percentage base
saturation of soils, and probably the proportion of the various bases present in the exchange
complex and in the soil solution, are primary factors which directly influence plant growth on
acid soils. This influence on plant growth comes from decreasing H, Al and Mn toxicities and
it is only in the absence of these toxicities that Ca saturation becomes a useful measure of
Ca availability to the plant (Adams 1984).
The ratio of Ca to Mg has been suggested for diagnostic purposes on the basis that it takes
into consideration the competing effect of Mg on Ca availability (Carter et al. 1979, McLean
1981). However, experimental findings are conflicting in this respect because the Ca:Mg
ratio overlooks the contribution of other cations, particularly Al in acid soils and Na in sodic
soils (Bruce 1999). So it is not precise to talk only about the Ca:Mg ratio when we discuss
plant nutrition because almost all cations tend to interact and compete against one another
for uptake by roots. For example, because K+ ion absorption by plant roots is affected by the
activities of other ions in the soil solution, some researchers prefer to use the ratio
represented by the following equation:
[K +]
2+
2+
[ Ca ] + [ Mg ]
rather than the K concentration to indicate the available K level in the soil solution (Brady &
Weil 2002). The major portion of cations that is absorbed by roots comes from the soil
solution. However, the relative concentration of cations in soil solutions may bear no
relationship to relative amounts of exchangeable cations. For example, sodium is often the
dominant cation in soil solution in spite of a low exchangeable amount. For this reason it has
been suggested that relative affinities of ions for exchange sites are more important than
concentration (Bell & Gillman 1978). Table 4 shows a general guide for the desirable
proportion of different cations ratio that suits different plants (Abbott 1989).
Table 4. Desirable proportion of different
cation ratio that suits different plants
(after Abbott 1989)
Cations
% CEC
Ca
Mg
K
Na
Al
65-80
10-15
1-5
0-1
<5
20
UNDERSTANDING SOIL ANALYSIS
As far as plant nutrition is concerned, there is no universal or unique Ca:Mg ratio because:
x
Plants differ in their absorption of Ca and Mg
x
Ca and Mg have different affinity for clay minerals which affects their exchangeability
and removal (leaching) rates
x
Soils differ in their content of Ca and Mg.
Some duplex soils in WA show different Ca:Mg ratios for topsoils and subsoils. For example,
a soil from Nyabing may have a 4.61 Ca:Mg ratio for the topsoil and half as much for the
subsoil. Whereas some of the sodic soils from Salmon Gums have 12.20 exchangeable
sodium percentage (ESP) and 0.92 Ca:Mg ratio at the topsoil, while in the subsoil the ESP
increases to 46.30 and the Ca:Mg ratio decreases to 0.26.
It is desirable to have Ca as the dominant cation on the exchange complex (Bruce 1999), but
Al may be dominant in acid soils and Na in sodic soils.
The Ca:Mg ratio is of greatest importance for good soil structure, particularly in clayey and
loamy soils. Bearing in mind that it is difficult to determine an optimum ratio for Ca:Mg, many
soil laboratories show in their soil report an ‘ideal’ ratio, based on the chemical analysis of
individual soils. It should not be used as the basis for making fertiliser
recommendations, but simply as supplementary diagnostic information.
Gypsum, lime and dolomite selection for particular soils
Lime and dolomite have given similar responses at most sites tested (Cregan et al. 1989). It
has been suggested that yield response was due to the lime effect on soil pH which allowed
the plant to better exploit subsoil Mg and not to the Mg in dolomite. The selection of one or a
combination of the materials for a particular soil should be based on the levels of calcium and
magnesium found and on pH. The quality of lime, dolomite and gypsum vary depending on
many factors. Purity, low salinity and absence of heavy metals are important in good quality
materials. Ca should make up the following amount of the pure chemical component as*:
Gypsum (calcium sulphate) .........................................
23.2% calcium, 18.6% sulphur
Lime (calcium carbonate) ............................................
40% calcium
Dolomite (calcium/magnesium carbonate) ..................
21.7% calcium, 13.2% magnesium
*
Calculations are based on pure chemical formula. When calculated on commercial formula the impurities
would reduce the percentage.
Note that:
x
Soils low in both calcium and magnesium are often lighter sandy soils and may require
amelioration with dolomite.
x
Soils low in calcium with high magnesium require lime or gypsum or a combination of
both.
The addition of gypsum to soil will increase calcium, while at the same time displacing some
magnesium. It is important to maintain a healthy ratio for good soil structure.
Some researchers have reported a decrease in lupin yield immediately after application of
lime or gypsum. The causes of yield reduction are not fully understood due to contradictory
21
UNDERSTANDING SOIL ANALYSIS
research results (Hamza & Anderson 2001), however the following causes have been
reported:
x
Competition between calcium (from gypsum or lime) and soil potassium
x
Iron deficiency due to increasing soil pH after liming (Alami et al. 1998; Foy 1997;
Birchall et al. 1995)
x
An osmotic effect on lupin yield due to gypsum application (McLay 1997; Tang et al.
1995).
It has been recommended by some researchers that lupins should not be sown for at least
two years after lime or gypsum applications (Tang et al. 1995; Hamza & Anderson 2001).
4.
Soil reaction, pH
The H+ concentration in soil, as well as in plant fluids, is generally low and for this reason is
expressed in terms of a logarithmic scale, or pH. The pH scale was suggested in 1909 by
SPL Sorensen as a simple numerical expression of acidity of a solution. It has proven its
importance in biology and is still in common use. The pH is defined as the logarithm of the
reciprocal of H+ ion concentration in solution: pH = log [1/H+]. Pure water is neutral because
it contains an equal amount of hydrogen ions (H+ = 10 7) and hydroxyl ions (OH = 10 7).
Thus the ion product of the concentrations of H+ and OH ions is a constant equal to 10 14.
Since the product of the concentrations of H+ and OH ions must be equal, the increase in
one of them must be accompanied by a decrease in the other. Thus, if the H+ concentration
in an acid medium is 10 5, the pH is five, if it is 10 9 in an alkaline medium, the pH is nine. A
soil pH 7 is regarded as neutral and a pH less than 7 is acidic, while a pH greater than 7 is
alkaline.
The concentration of H+ ions in the soil represents actual acidity, while potential acidity also
includes adsorbed H+ on the soil exchange sites (clay plus organic matter). Soils differ in
their buffering capacity (the ability of a soil to resist changes in pH) for acidity, depending on
the content of colloids. The higher the colloidal content of the soil, the higher the buffering
capacity and the greater the difficulty for changing pH (clay soils have much higher buffering
capacity than sandy soils).
The pH values of soils can differ widely from values of about three to as high as ten. In
alkaline soils in particular, very high pH is commonly caused by the presence of weak acids
(HCO3 ) and strong bases (Na+ or K+). The pH at the root surface (rhizosphere) may be as
much as one unit different from that of the bulk soil, depending on OH or H+ effluxes from the
roots (Nye 1977). If the plant absorbs more cations (positively charged ions such as K+) than
anions (negatively charged ions such as Cl ), it releases H+ into the soil to maintain a
negative potential across the root cell membrane (around -150 mV). If the plant absorbs
more anions than cations, it releases OH or HCO3 into the soil to keep cell potential
negative. In most cases, plants absorb more cations than anions and as a result release H+
ions to the soil lowering the value of pH at the root surface. The type of N fertiliser (whether
NH+4 or NO3 ) plays an important role in this regard.
The mix of plant species that dominates a landscape under natural conditions often reflects
the pH of the soil (Brady & Weil 2002). The degree of soil acidity or alkalinity, expressed as
soil pH, is a master variable that affects soil chemical, biological and indirectly its physical
properties. Soil pH greatly influences the availability for root uptake of many elements,
whether they are essential or toxic to the plant. A generalised idea of the relationships is
shown in Figure 19. High H+ concentration (low pH value) favours the weathering of minerals
resulting in a release of various ions such as K+, Mg2+, Ca2+, Mn2+, Cu2+ and Al3+. The
solubility of salt minerals including carbonates, phosphates and sulphates is higher in the
22
UNDERSTANDING SOIL ANALYSIS
lower pH range. The release and thus toxicity of Al in various forms from clay minerals is
also pH dependent. However, Al solubility increases exponentially when soil pH drops below
4.5 (see Figure 19).
Soil pH also affects the mobility of many pollutants in soil by influencing the rate of their
biochemical breakdown, their solubility and their adsorption on colloids.
Figure 19 shows that, when the correlation between soil pH and nutrient availability is
considered as a whole, a pH range of about 5.5 to perhaps 7.0 seems to be best to promote
the availability of plant nutrients. In short, if the soil pH is suitably adjusted for phosphorus,
the other plant nutrients, provided they are present in adequate amounts, will be satisfactorily
available in most cases (Brady & Weil 2002).
Fungi
Bacteria
and actinomycetes
N
K
S
Ca
Mg
P
Fe
Mn
Mo
Cu
B
Zn
Figure 19.
The relationships between soil pH, availability of plant nutrients and the activities of
some soil micro-organisms. The width of the bands indicates the relative microbial activity or
nutrient availability. The jagged lines between the P band and the Ca, Al and Fe bands represent
the effect of these metals in restricting the availability of P (Courtesy of Brady & Weil 2002).
23
UNDERSTANDING SOIL ANALYSIS
There are three methods in determining soil pH:
x
In water (pHw), where pure water is used to make a soil suspension. In this method
two major drawbacks may affect the pH reading: firstly, water dilutes the soil solution
leading to values that are 0.2 to 0.5 units higher (for acid soils) than would be
measured in the undiluted field soil solution. Secondly, small variations in soluble salt
content of soil due to addition of fertilisers, or evaporative salt accumulation, can result
in readings that differ by as much as 0.5.
x
In CaCl2 (pHCa), using a weak, unbuffered salt solution instead of pure water to make
the soil suspension. This would overcome the two problems mentioned in the previous
method. A 0.01M CaCl2 solution is used to provide a background electrolyte
concentration that is similar in ionic strength to the soil solution in the field. The Ca2+
added in the solution forces a portion of the exchange acidity to move into the active
pool, giving CaCl2 readings that are typically 0.2 to 0.5 units lower than pHw for the
same soil (see Table 5 of conversion below).
x
In KCl (pHKCl), the soil is mixed with a more concentrated neutral salt solution such as
1M KCl. This solution is concentrated enough to displace into the soil solution all the
exchangeable acidity on the soil CEC by K+. The reading gives about one unit lower
than the pHw, although the difference may be greater for soils with high pH level. The
one unit difference indicates that the exchangeable pool is commonly about 10 times
as great as the active pool of soil acidity.
When determined in the laboratory, the pH value is often measured by pH meter using a
glass electrode on a 1:5 extract (Rayment & Higginson 1992a).
To convert pHw to pHCa or vice versa (in a 1:5 soil solution) the following equation is used in
general: pHw = pHca + (0.5 to 1.0). For more detailed conversion, use Table 5.
Table 5. Conversion of pHca to pHw
pHCa
Conversion factor
add or subtract*
pHw
7.3-7.8
0.5
7.8-8.3
6.6-7.2
0.4
7.0-7.6
6.0-6.5
0.5
6.5-7.0
5.4-5.9
0.6
6.0-6.5
4.4-5.3
0.7
5.1-6.0
1.0-4.3
0.8
4.9-6.0
3.8-4.0
0.9
4.7-4.9
< 3.7
1.0
< 4.7
x
To convert pHw to pHCa subtract the value of the conversion factor from the measured pHw value. For
example if the measured pHH2O value was 6.8, then the pHca value should be 6.8 - 0.5 = 6.3. To convert
the value of pHCa to pHw add the conversion factor to the measured HCa values.
In general, each pH unit measured in water below 7.0 means that around 15 per cent of CEC
(cation exchange capacity) is occupied by H+. For example H+ ions at pHW = 7.0, occupies 0
per cent of the CEC while at pHW = 6.0, it occupies 15 per cent. Too much acidity decreases
the nutrient-holding capacity of the soil because H+ replaces other essential elements.
However, acidification is a naturally occurring process in humid regions where the rainfall is
sufficient to thoroughly leach basic cations from the soil profile. By contrast, in low rainfall
(dry) regions such as the inland cropping regions of Western Australia, leaching is much less
extensive, allowing soils to retain enough Ca, Mg, K and Na ions to prevent a build-up of acid
24
UNDERSTANDING SOIL ANALYSIS
cations, thus enhancing the process of alkalisation (that is, pH >7, Brady & Weil 2002).
However, in arid areas, especially if extensively cropped, soil acidity may occur. The main
causes beside parent materials are the high amount of basic cations exported from the soil
and the acidity stemming from applying NH4 fertilisers.
It should be mentioned that soil pH is only a measure of hydrogen ions, not an indication of
soil fertility or mineral balance by itself. The ideal soil pH for most crops is from 5.8-6.5 (in
CaCl2).
Sources of soil acidity are:
x
Acids from plant roots
In most cases plants absorb more cations than anions and release H+ into the soil to keep
the electrical potential negative across cell membranes (Clarkson 1984). Nutrients are
absorbed by plant roots along their electrochemical gradient (electrical charge of the ion plus
the concentration of the ion). Plant roots usually maintain a negative electrical potential
across their root surface membrane (between root cell cytoplasmic solution and soil solution)
which facilitates the absorption of cations.
x
Soil CO2
Root respiration and the decomposition of soil organic matter by micro-organisms produces a
high level of CO2 in soil air. CO2 dissolves in water to form carbonic acid (H2CO3). However,
H2CO3 is a weak acid, and its contribution to soil acidity is very small when the soil pH is
much below 5.
x
Acids from microbial activities
As microbes break down soil organic matter, many organic acids are released into the soil.
Some of these organic acids, such as citric or malic acids, have a low molecular weight and
weakly dissociate, thus producing only a small effect on soil pH. Others are more complex
and stronger acids, such as the carboxylic and phenolic acid groups contained in humic
substances.
x
Accumulation of organic matter
Organic matter tends to acidify the soil in two ways:
(i)
It forms soluble complexes with non-acidic cations (basic cations) such as Ca and Mg,
thus facilitating the loss of these cations by leaching.
(ii)
Organic matter itself is a source of H+ ions because it contains numerous acid
functional groups from which these ions can dissociate.
x
Oxidation of nitrogen (nitrification)
N is released from fertilisers into the soil solution as NH4+ ions. NH4+ Ions in turn, are subject
to oxidation, forming nitrate ions (NO3 ). The oxidation process is usually carried out by
specific soil bacteria, but can also take place by a purely chemical reaction termed
nitrification. The reaction releases two H+ ions for each NH4+ ion oxidised:
NH4+ +
2O2
H2O
+
H+
+
H+
+
NO3-
Dissociated
nitric acid
25
UNDERSTANDING SOIL ANALYSIS
The long-term balance between the processes of acidification (production of H+) and
alkalisation (consumption of H+) in soil systems reflects the pH level of the soil. Table 6
summarises these two major processes.
Table 6.
Processes of acidification and alkalisation in soil systems
Acidification processes
Alkalisation processes
Formation of carbonic acid from CO2
Input of bicarbonates or carbonates
Acid dissociation as:
RCOOH
RCOO- + H+
Anion protonation such as:
RCOO- + H+
RCOOH
Oxidation of N, S and Fe compounds
Reduction of N, S and Fe compounds
Cation uptake, leaching and precipitation
Anion uptake by plant
Accumulation of acidic organic matter
Cations weathering from minerals
De-protonation of pH-dependent charges
Protonation of pH-dependent charges
Liming
Liming is the application of alkaline materials that provide conjugate bases of weak acids,
such as carbonate (CO 3) and sometimes hydroxide (OH ). These are anions capable of
reacting with H+ and converting it to weak acids such as water. When calcitic lime (CaCO3)
is applied to an acid soil, each molecule neutralises two H+ ions:
Usually, liming materials are supplied in their calcium and magnesium forms (see Table 7)
and referred to as agricultural limes (lime sand, limestone or dolomite). Sometimes calcium
oxide (CaO), also called burnt lime, is used to give OH upon dissolving in water (hydrolysed).
OH ions then react with H+ and are converted to water. CaO is corrosive and special
precautions should be taken when using it. This includes using corrosion-resistant
containers and protective clothing. However, it has more neutralising power per unit mass
than the calcite limes. Furthermore, the pH of calcium hydroxide in water is about 11, which
can be dangerous to eyes and skin compared to a pH of about 8.3 for calcium carbonate,
which is about the same as seawater. Dissolving calcium oxide in water generates a lot of
heat, which can burn or cause steam which may blow the caustic solution into the face and
eyes. In practice it is not widely available in Western Australia.
Because lime is needed in large quantities, the cost of transporting it becomes an important
issue in choosing the source of lime needed. However, the concentration of Ca and Mg in
the soil is also important in choosing the liming material. If Mg is deficient in an acid soil,
then dolomite, Ca Mg(CO3)2, can be used. Pure dolomite contains 21.7 Ca and 13.2 Mg by
weight. However, commercial samples will be less pure than this. The nutrients supplied to
plants by the liming material should not be overlooked. In some highly weathered soils like in
Western Australia, small amounts of lime may improve plant growth, more because of the
enhanced calcium or magnesium nutrition than from a change in pH.
Table 7 shows different liming material used in Western Australia. Calcitic limestone and
dolomite are the most commonly used.
26
UNDERSTANDING SOIL ANALYSIS
Table 7. Common types of liming material used in Western Australia
Lime type
Calcitic limestone (lime sand)
Chemical formula
CaCO3
2
Dolomitic limestone
CaMg (CO3)
Burned lime (oxide of lime)
CaO, MgO
% CaCO3
equivalent
Solubility
(g/L)
100
0.014
95-108
0.32
178
1.85
How much lime is needed?
The action of liming is not instantaneous, but gradual depending on the solubility of lime. It
may take weeks for hydrated lime, to a year or more for calcitic lime, to show some results.
As Ca and Mg are removed from the soil by crops, or by leaching, they are replaced by acid
cations and soil pH is continuously reduced. Eventually, another application of lime is
needed depending on the starting pH and if the soil is in a maintenance phase or a recovery
phase. However, in the low rainfall areas where both removal of Ca and Mg by crops and
leaching is low, lime application is needed at longer intervals, as compared to high rainfall
areas. In addition to economic considerations the amount of lime to add depends on:
x
change required in the pH or exchangeable Al saturation
x
buffering capacity of the soil
x
type of liming material
x
purity of neutralising value and fineness of the liming material.
Calculating how much lime is needed.
Most laboratories have buffer curves for the major types of soils in their service area to
calculate how much lime is need to raise pH from the current value to the desired value. A
calculation of the lime requirement similar to the example given below is done by the soil
testing laboratory.
In the Nungarin Shire we have a loamy sand (a common soil type in WA) with a bulk density
value of 1.3 mg/m3 and soil pH 4.9 and we want to increase the soil pH to 6.5. From the pH
buffering curve of the soil we find that we need 2.1 cmolc of lime/kg soil. How much
commercial lime with 90 per cent purity is needed to supply 2.1 cmolc lime/kg for 1 ha?
x
Each CaCO3 molecule neutralises 2H+ ions (CaCO3 + 2H+ Ca2+ + CO2 + H2O).
x
The molecular weight of CaCO3 = 100 g/mole, then the mass of 2.1 cmolc of pure
CaCO3 is:
(2.1 cmolc /kg soil) X (100 g/mol CaCO3) X (1 mol CaCO3/2 cmolc) X (0.01 molc/ cmolc)
= 1.05 g CaCO3/kg soil.
x
The weight of 1 hectare of soil with bulk density of 1.3 Mg/m3 to 0.15 m depth is:
10,000 m2/ha X 0.15 m X 1.3 Mg/m3 = 1950 Mg soil/ha (or 1,950,000 kg soil/ha).
x
Then the amount of pure CaCO3 needed per hectare:
(1.05 g CaCO3 /kg soil) X 1,950,000 kg soil /ha) = 2,047,500 g CaCO3 /ha or
2,047 kg pure CaCO3 /ha.
Since the purity of the limestone is 90 per cent, then we need:
(2047 X 100) / 90 = 2,274 kg or 2.274 t/ha of commercial limestone.
27
UNDERSTANDING SOIL ANALYSIS
Note that not all limestone will react with soil in the field, so we might add more than the
calculated material by one and a half to two times. It is better to split the application into two
halves, adding the second half after a few years. Refer to a soil adviser for recommendation.
The main problem with calcitic lime is its low solubility (0.014 g/L), which means it moves
very slowly in soil, so injecting lime to reduce subsoil acidity may be desirable. Development
of tools to inject and mix lime into the subsoil is an important issue in addressing subsurface
soil acidity.
Role of aluminium in soil acidity
Aluminium plays a central role in soil acidity and is responsible for much of the deleterious
impact of soil acidity on plants. Most soil minerals contain Al as aluminosilicate and
aluminium oxides (see Figure 7). When H+ ions are adsorbed on a clay surface they usually
do not remain as exchangeable cations for long, but instead they attack the structure of the
minerals, releasing Al+3 ions in the process. The Al+3 ions then become adsorbed on the
colloid’s cation-exchange sites. These exchangeable Al+3 ions in turn, are in equilibrium with
dissolved Al+3 in the soil solution. The exchangeable and soluble Al+3 ions play two critical
roles in the soil acidity:
(i)
Aluminium is highly toxic to most soil organisms and to plant roots.
(ii)
Aluminium ion Al+3 reacts with water and splits to produce H+ (hydrolysis). In fact, a
single Al+3 ion can release up to three H+ ions in a reversible reaction series which
proceeds in a stepwise fashion. For this reason Al+3 and H+ together are considered
acidic cations.
The positively charged hydroxy aluminium ions (AlOH2+ and Al(OH)+2) form large polymers,
which are tightly bound to the colloid’s negative charge sites, reducing the effective CEC of
the soil. At the same time the formation of non-soluble Al(OH)30 makes more negative sites
on the colloids become available for cation exchange. This is why the cation exchange
capacity of the soil increases as the pH is raised from 4.5 to 7.0.
Pools of soil acidity
Four pools of acidity are common in soils:
(i)
Active acidity: Due to the H+ and Al3+ ions in the soil solution (this is the pH measured
in field soil tests).
(ii)
Exchangeable acidity: Involving the exchangeable Al3+ and H+ that are easily
exchanged by other unbuffered salts, such as KCl. Once released to soil solution, the
aluminium hydrolyses to form additional H+ thus increasing soil acidity. The
exchangeable acidity in strongly to moderately acid soils is commonly thousands of
times that of active acidity. The limestone needed to neutralise this type of acidity is
commonly more than 100 times that needed to neutralise active acidity. However,
since the common type of clay in WA soils is kaolinite, the exchangeable acidity is
lower than that of both smectites and vermiculites (laboratory tests measure both active
and exchangeable pH).
(iii)
Residual acidity: This is commonly far greater than either the active or exchangeable
acidity. It may be 1000 times greater than the soil solution or active acidity in a sandy
soil and may be up to 100,000 times greater in a clayey soil high in organic matter. It is
generally associated with hydrogen and aluminium ions that are bound in
non-exchangeable form by organic matter and clay. As the pH increases, the bound
hydrogen dissociates and the bound aluminium ions are released and precipitate as
amorphous Al(OH)3.
28
UNDERSTANDING SOIL ANALYSIS
(iv) Potential acidity from reduced sulphur. This occurs only in certain acid sulphate soils
and arises from the oxidation of sulphur compounds.
For most soils, except potential acid-sulphate soils, the total acidity that must be overcome to
raise the soil pH to a desired value equals (Active acidity + exchangeable acidity + residual
acidity).
Most farmers in Western Australia address soil acidity by applying one to two tonnes per
hectare of lime sand to the topsoil. Some farmers inject this amount into the subsoil. Using
an acidity tolerant species is a good way of co-existing with soil acidity.
5.
Sodicity
Sodic soil is non-saline soil containing sufficient exchangeable sodium to adversely affect
crop production and soil structure under most conditions of soil and plant type.
The stability of soil aggregates when wet is governed by forces operating both within the clay
particles and from the surrounding soil solution. The presence of sodium ions on clay
surfaces creates very high internal swelling pressures. This swelling keeps the clay particles
apart and causes aggregates to break down and the clays to disperse. The presence of
calcium ions on clay surfaces has the opposite effect.
Sodic soils are widespread in arid and semi-arid regions. In WA, 26% of soils are
predominantly sodic. Sodic soils are subject to severe structural degradation and restrict
plant performance through poor soil-water and soil-air relations (Rengasamy 1998). Sodicity
is linked to salinity because it is associated with salt accumulation in the soil profile when
evapotranspiration exceeds precipitation. When the salts present are predominantly sodium
salts, the soil becomes sodic and plant growth is affected indirectly by degrading the physical
behaviour of soils. Sodic clay particles when separated by hydration (wetting), repel each
other and remain dispersed when wet. They are then free to block soil pores, thus
significantly restricting water and gas movement into the soil. The dispersion of sodic clay
and the swelling of soil aggregates can destroy soil structure, and so reduce porosity and
permeability of soils and increase the soil strength even at low suction (that is, high water
content). Plants cannot grow well because sodic soils are either too wet directly after rain or
too dry within a short period after rain, so that duration of the soil water content range
favouring plant growth is limited. Waterlogging, poor crop or pasture emergence and
establishment, and soil erosion may indicate that the soil is sodic. However, these
symptoms may be caused by several other problems such as soil compaction, and so a
specific test for sodicity is needed. The coherence of soil aggregates in salt free water is
almost the only way to test soil sodicity in the field, because clay is spontaneously dispersed
by water when it is sodic.
Soil sodicity can be assessed by measuring the exchangeable sodium percentage (ESP):
ESP = (Exchangeable sodium/Cation exchange capacity) 100
where CEC refers to the sum of the cations (Ca, Mg, Na and K, etc.).
When ESP is greater than 15 the soil is regarded as sodic. However, sodic soils are
sometimes described and classified according to their behaviour instead of using analytical
criteria, because soils may exhibit sodic properties even though they have relatively low ESP
values. This is the case in Western Australia where the soil is regarded as sodic when ESP
is greater than 6.
29
UNDERSTANDING SOIL ANALYSIS
Sodium absorption ratio (SAR) also can be used as indication of soil sodicity; it shows the
relation between soluble sodium and soluble divalent cations which can be used to predict
the exchangeable sodium fraction of soil equilibrated with a given solution. It is defined as:
SAR
[ Na ]
[Ca 2 ] [ Mg 2 ]
where the concentrations, denoted by bracket, are expressed in mmoles per litre.
Commonly the SAR of water is adjusted for precipitation or dissolution of Ca2+ that is
expected to occur where water reacts with alkaline earth carbonates within a soil.
Classically, a sodic soil will respond to gypsum applications. By replacing sodium with
calcium on the exchange sites, the clay particles will aggregate more readily and these
aggregates will not cause the problems associated with dispersive, high ESP clays.
Relationship between sodicity and soil pH
In general, high sodium concentration (high ESP) is usually associated with high soil pH
because both Ca2+ and Mg2+ ions are commonly precipitated out of soil solution by carbonate
and bicarbonate ions. A general relationship is shown in Table 8 (Gupta & Abrol 1990).
Table 8. Relationship between soil pH and
exchangeable sodium percentage, ESP
pH in 1:2 soil:water suspension
ESP
8.2-9.0
9.0-9.4
9.4-9.6
9.6-9.8
9.8-10.0
>10.0
<20
21-35
35-50
50-65
65-85
>85
6.
Salinity (EC)
Salts accumulate in soils when, in general, rainfall is not enough to leach the salt out of the
rooting zone or the drainage is not sufficient. Accordingly, salt has more chance to
accumulate in low rainfall than the high rainfall areas. However, there can be exceptions in
some high rainfall zones (e.g. Ferdowsian & Greenham (1992) found that at Upper Denmark
(750 mm rainfall zone) high levels of salt stored in the soil profile.)
There are two main effects of salinity on crops. The first is the osmotic (total soluble salt)
effect and the second is the specific ion effect. In the osmatic effect, the plant roots’ ability to
absorb water decreases with increasing soil salinity, then stops altogether when the osmotic
potential equals that inside the plant root. If soil solution salinity increases further, water
moves along a water potential gradient from the root to the soil and roots start shrinking
(Hamza et al. 2006). In the specific ion effect, ions such as Na+, Cl , H4BO4 and HCO3 , are
quite toxic to many crops when they exist in high concentration. Na+ for example causes soil
dispersion and compete with K+ in the process of transport across the cell membrane during
uptake (Brady & Weil 2002)
Salinity is measured as electrical conductivity (EC) by a conductivity meter at 25qC on a 1:5
soil:water suspension (Rayment & Higginson 1992b). To relate the effect of salinity on plant
roots under field conditions, the suspension salinity (EC1:5) must be converted to saturation
salinity (ECe):
ECe (mS/cm) = EC 1:5 (mS/cm) X conversion factor
30
UNDERSTANDING SOIL ANALYSIS
where the subscript e refers to the saturation extract of the soil. The conversion factor
depends on soil texture. The relationship in Table 9 can be used for converting EC1.5 to ECe.
Table 9. factors for converting EC1:5 to ECe for
different soil texture
Soil texture
Conversion
factor
Sandy soils
Sandy loam soils
Sandy clay loam soils
Silty loam, silty clay soils
Clay soils
21
12
9
8
6
These values are based on field experience in Western Australia and may change to some
degree from area to area.
Salinity is measured in a number of ways:
x
mS/cm (milliSiemens per centimetre), which is equal to dS/m (deciSiemens per
metre)
x
mS/m, which is equal to mS/cm X 100
x
PS/m (microSiemens per metre), which is equal to mS/m X 1000.
The following conversions might help in understanding the relationship between these units
(Moore 2004):
1 mS/m
= 10 Ps/cm = 1000 PS/m
= 0.01 dS/m
= 0.01 mS/cm
= 5.5 mg/L or ppm
dS/m = mS/cm = mmho/cm
concentration of salt (mg/L) = 550* X EC
* This factor is for Western Australian soils. For other soils this factor may be as high as 640.
To convert EC1:5 to total soluble salt (TSS) in the soil:
TSS (mg/L) = EC1:5 X 550
Where EC1:5 is in dS/m and TSS is in mg/kg (ppm)
The Soil and Water Salinity Calculator published by the Department of Agriculture and Food,
provides a handy conversion table for the various units.
Crops differ in their tolerance to salinity, but in general soil ECe above 4 mS/cm is regarded
as salt affected soil. Table 10 gives an estimate for yield reduction due to salinity under
different salinity levels (electrical conductivity of saturated soil extract) for some crops (Ayers
1977).
31
UNDERSTANDING SOIL ANALYSIS
Table 10. Estimation for yield reduction at different salinity levels
Expected yield reduction at ECe indicated
Crop
7.
0%
10%
25%
50%
Barley
8.0
10.0
13.0
18.0
Cotton
7.7
9.6
13
17.0
Wheat
6.0
7.4
9.5
13.0
Safflower
5.3
6.2
7.6
9.9
Soybean
5.0
5.5
6.2
7.5
Sorghum
4.0
5.1
7.2
7.2
Rice
3.0
3.8
5.1
7.2
Beans (field)
1.0
1.5
2.3
3.6
Cation Exchange Capacity (CEC)
CEC is expressed as the number of moles of positive charge adsorbed per unit mass
(cmol(+)/kg = centimoles/kg). Some laboratories still use the older unit milliequivalent per
100 grams (me/100 g) which gives the same value as cmol(+)/kg (1 me/100 g = 1 cmol(+)/kg).
For example, a soil which has a CEC of 12 cmol(+)/kg indicates that 1 kilogram of the soil can
hold 12 centimole of H+ ions, and can exchange this amount of charges for the same number
of charges from any other cation. This means that the exchange reaction takes place on a
charge-for-charge (not an ion-for-ion) basis. Accordingly, two Na+ ions can exchange for one
Ca2+ ion. This also may indicate to some degree how strongly an individual cation is held on
the soil colloids. For example, Al3+ is held on the soil exchange sites more strongly than Ca2+
and this in turn more strongly than Na+. So the order of adsorption strength can be:
Al3+ > Ca2+ > Mg2+ > K+ = NH4+ > Na+.
This order is important when a particular cation used as soil amendment is replacing another
cation on the exchange complex.
CEC can be determined using different methods. However, in all methods the adsorbed
cations must be replaced by a single exchanger cation such as Ba2+, NH4+ or Sr2+ and then
the CEC is calculated either from the amount of the exchanger cations used for replacement
or from the amounts of each of the replaced cations originally held on the soil exchange sites
(usually Ca2+, Al3+, Mg2+, K+ and Na+). Most laboratories determine CEC in a buffered
solution using one M NH4Cl at pH 8.5 in 60 per cent ethanol, or ammonium as the exchanger
cation at pH 7.0, or barium as the exchanger cation at pH 8.2. If the soil pH is less than the
pH of the buffered solution then the pH-dependent exchange sites that would become
negatively charged at pH 7.0 or 8.2 will be measured too.
The CEC of a given soil is determined by the relative amount of different colloids in that soil
and by the CEC of each of these colloids. The major soil colloids are clay and organic
matter. Silt may contribute a little to soil CEC, while the contribution of sand is very small
and in most cases negligible. In sandy soils most of the CEC comes from the clay and
organic fractions of the soil. Table 11 shows the CEC of some of the soil constituents.
32
UNDERSTANDING SOIL ANALYSIS
Table 11. CEC of some soil constituents (cmol(+)/kg)
Soil constituents
CEC, cmol(+)/kg
Kaolinite
5-15
Chlorite
10-40
Illite
20-50
Transitional minerals
40-80
Montmorillonite (smectite)
80-120
Vermiculite
100-150
Organic material (humic acid)
200-500
In Western Australia many duplex soils have sandy topsoil and clayey subsoil. These soils
are commonly low in CEC, particularly in the topsoil, because they are often composed of
leached sands that might contain some highly weathered soils with kaolinite as the dominant
clay silicate (Table 12). The magnitudes of CEC, ionic composition and base saturation of
soils are related to organic matter, clay content, pH and soil origin.
Table 12. Subsoils from Geraldton (north), Wickepin (middle) and Esperance (southeast) in Western Australia showing some soil properties
Soil property
CEC, cmol(+)/kg
Base saturation, %
Geraldton
Wickepin
2.81
2.88
67
67
Esperance
32.39
100
Organic matter, %
0.71
0.59
0.62
pHw
5.40
5.40
9.00
Exchangeable Ca, %
40.00
38.20
9.60
Exchangeable Mg, %
13.60
13.80
37.10
Exchangeable K, %
3.70
2.20
4.70
Exchangeable Na, %
3.10
6.20
46.30
Exchangeable H, %
33.00
33.00
0.00
Table 12 shows that the value of base saturation gives direct insight into the amount of
acidity/alkalinity in the soil and which cations are deficient or in excess. If the soil base
saturation is low, as in the Geraldton or Wickepin soils, it is difficult to get a good yield before
increasing the base saturation (removing excess acidity). Removing excess acidity through
application of lime would in most cases improve the soil base saturation.
The CEC will determine the fertiliser practices that are appropriate and the amount of
nutrients needed to correct imbalances. It will take more fertiliser to balance high CEC clay
than low CEC sand. Sand will need smaller but more frequent applications of nutrients.
The ideal CEC range for soil fertility is around 10-18 cmol(+)/kg, however most WA soils are
sandy and have low to very low CEC. In duplex sand over clay or loam soils, ripping the soil
may improve CEC if the clay is not too deep. Hamza and Anderson (2002) were able to
increase clay content of the topsoil by about 10 per cent in Merredin through ripping.
33
UNDERSTANDING SOIL ANALYSIS
Exchangeable cation determinations
Cations such as Ca, Mg, K and Na are usually measured by one of three procedures:
x
1M NH4Cl at pH 7.0 Used for neutral soils (pHw between 6.5 and 8). Here, cations
Ca, Mg, Na and K are measured by ICP-AES (inductively coupled plasma atomic
emission spectrometry). Soluble salts must be removed from soils with EC1:5
> 20 mS/m by washing with glycol-ethanol.
x
0.1 M BaCl2 (unbuffered) Used for acid soils only (pH <6.5). This method is based on
a modification of Gillman et al. (1984). Cations Ca, Mg, Na, K, Al and Mg are
measured by ICP-AES. Soluble salts are removed from soils with EC(1:5) > 20 mS/m
by washing with glycol-ethanol.
x
1M NH4Cl, pH 8.5 - Used for calcareous soils. This is a modification of Rayment and
Higginson (1992c). Ca, Mg, Na and K are measured by flame AAS (atomic absorption
spectrophotometry).
Conversions
To convert cmol(+)/kg to t/ha, use the following equation (Hazelton & Murphy 2007):
1 cmol (+) /kg = (0.001 X MW X D X BD)/C
where MW is the molecular weight of the cation or anion in grams, D is the depth of the soil
in cm, BD is the bulk density of the soil in g/cm3 and C is the charge of cation or anions.
For example, if you want to convert the Ca in 1 cmol (+) /kg to t/ha with soil depth of 10
centimetres and BD of 1.4 g/cm3, then:
1 cmol (+) /kg of Ca = (0.001 X 40.08 X 10 X 1.4)/2 = 0.28 t/ha of Ca.
8.
Organic Matter (OM)
Soil organic matter comprises all the living, dead and decomposing plants, animals and
microbes along with the organic residues and humic substances they release. Organic
matter is the major pool of terrestrial carbon in soil and provides major soil energy and
nutrient reserves (Carter 1996). The type of organic matter is also important, for example
readily oxidisable soil organic matter seems to be more relevant than total organic matter in
determining mechanical behaviour of the soil (Ball et al. 2000).
Three different types of soil organic matter are recognised (Lickacz & Penny 2001):
x
x
x
Raw plant residues and micro-organisms (1-10 per cent)
’Active’ organic fraction (10-40 per cent)
Resistant or stable organic matter (40-60 per cent) also referred to as humus.
Soil organic matter is a complex mixture of materials that vary in size, chemistry, degree of
decomposition and interaction with soil minerals.
Besides the nutritional value of soil organic matter, it provides a good way of protecting soil
from erosion. For example, plant residues on the soil surface help reduce water runoff, thus
decreasing soil erosion. It also reduces wind erosion by providing plant cover. Removal or
burning of residues predisposes the soil to serious erosion. It is highly recommended that
stubble is retained and not burned unless necessary, such as in case of epidemic disease.
34
UNDERSTANDING SOIL ANALYSIS
Micro-organisms (especially fungi) with the ’active‘ and some resistant soil organic
components, are involved in binding small particles into larger aggregates. Aggregation is
important for good structure, aeration, water infiltration and resistance to erosion and
crusting.
The resistant or stable fraction of soil organic matter contributes mainly to cation exchange
capacity and colour. This fraction of organic matter decomposes very slowly and therefore
has less influence on soil fertility than the ’active‘ organic fraction.
Organic matter in soil serves several functions, but from a practical agricultural standpoint, it
is important for two main reasons: ’nutrient recycling‘; and as an agent to improve structure,
maintain tilth, and minimise erosion.
The nutrient recycling function has two main actions:
x
Since soil organic matter is derived mainly from plant residues, it contains all of the
essential plant nutrients. Accumulated organic matter, therefore, is a storehouse of
plant nutrients. Upon decomposition, nutrients are released in a plant-available form.
x
The stable organic fraction (humus) adsorbs and holds nutrients in a plant-available
form.
Organic matter does not add any ’new’ plant nutrients, but releases nutrients in a plant
available form through the process of decomposition. In order to maintain this nutrient
cycling system, the rate of addition from crop residues and manure must equal the rate of
decomposition.
Soil organic matter will decline if the rate of addition is less than the rate of decomposition,
and conversely, soil organic matter will increase if the rate of addition is greater than the rate
of decomposition. The term steady state has been used to describe a condition where the
rate of addition is equal to the rate of decomposition. Fertiliser can contribute to the
maintenance of this nutrient recycling by increasing crop yields and consequently the amount
of residues returned to the soil.
Organic matter plays a vital role in biological, chemical and physical soil fertility. In dryland
areas such as Western Australia, where soil biological and physical fertilities are degraded,
the contribution of organic matter in invigorating soil micro-organisms and improving soil
physical properties, especially of topsoil, is very important. The evidence that soil structure
improves and becomes more resistant to degradation as soil organic matter content
increases is overwhelming (Cochrane & Aylmore 1994; Thomas et al. 1996; Hamza &
Anderson 2005). Maintaining an adequate amount of organic matter in the soil helps
regulate air and water infiltration, increases resistance to erosion, stabilises soil structure,
increases water and nutrient storage, decreases bulk density and soil strength, invigorates
soil micro-organisms and restores crop productivity to levels proportional to high-input
agricultural systems (Sparovek et al. 1999; Carter 2002). Losses of soil organic matter and
aggregate stability are regarded as standard features of non-sustainable land use
(Håkansson 2000; Carter 2002). The positive correlation between soil organic matter
content and physical properties is attributed to their mutual effects, where organic matter
binds soil particles (Figures 20 and 21), while soil aggregates protect organic matter from
decomposition (Carter 1996). This is related to the contribution of organic matter to wet
aggregate stability (Zhang 1994). Organic matter also retains a greater amount of moisture,
thus helping soil to rebound against compaction (Paul 1974).
Increase in soil organic matter is desirable because the various components provide a
multitude of beneficial functions for the soil, such as:
35
UNDERSTANDING SOIL ANALYSIS
x
Providing energy and nutrients to soil micro-organisms. Much of the impact of organic
matter on soil health is achieved by stimulating soil micro-organisms
x
Binding soil mineral particles (Theng & Oades 1982; Zhang 1994)
x
Reduction of aggregate wettability (Zhang & Hartge 1992)
x
Influencing the mechanical strength of soil aggregates, which is the measure of
coherence of inter-particle bonds (Quirk & Panabokke 1962).
x
Prevents transferring soil compaction to subsoil
x
Leads to better plant nutrition, ease of cultivation and seedbed preparation.
x
Greater aggregate stability, reduced bulk density, improved water-holding capacity,
enhanced porosity and earlier warming in spring.
Very low OM
High OM
Figure 20. Soil high in organic matter is granulated and has significantly
lower bulk density compared to soil low in organic matter.
Bacterial
cell
Figure 21. Organic matter (light colour) binds clay
particles (dark colour) hence improving
soil aggregation.
36
UNDERSTANDING SOIL ANALYSIS
Organic matter is estimated from measurements of organic carbon. Organic carbon is
usually determined, on soil ground to less than 0.15 mm, by Metson's colorimetric
modification of the Walkley & Black method. The procedure is based on oxidation of soil
organic matter by dichromate in the presence of sulphuric acid. In general, organic matter
contains around 50 per cent organic carbon, so multiplying organic carbon (OC) by two gives
a good estimation of organic matter. Most soil laboratories measure only the fine (less than
2 mm) or colloidal organic matter, which does not include bulk material like straw and roots.
It is therefore a good indicator of soil humus levels. In some soils the organic carbon may
contain large amounts of charcoal. Although charcoal is no longer actively decomposing
(non-labile) it can still play a role in holding and supplying nutrients and water for plant
growth. The lability of organic matter depends on the degree of decomposition, which
exposes more surface area for interaction. However, partially decomposed organic matter
provides an important source of energy and nutrients for soil micro-organisms and also helps
soil aggregation.
Soils with low organic matter require more artificial nitrogen than soils rich in organic matter.
Ideal organic matter content is around 4 to 6 per cent, but this is hard to achieve in the low
rainfall areas where the average organic matter is commonly in the range of 1 to 3 per cent.
The following sections discuss nutrients that have been proven essential for plants. There
are many published visual guides that illustrate deficiencies or toxicities (Snowball & Robson
1983; Bennett 1993).
9.
Nitrogen (N)
Extensive cropping has led to severe decline in soil N in cereal soils throughout Western
Australia (Strong & Mason 1999). To sustain high grain yield of the quality sought in
competitive markets, N must be supplemented as fertiliser. However, a soil test is of little
use in determining the soil N status accurately, or the plant requirement for fertiliser.
Measurement of total soil nitrogen or total organic carbon can give a general idea of N
status, but not an accurate idea of its availability to the plant. Soil tests along with other
information such as soil type, field history and crop yield potential can be used for
determining a recommendation for N fertiliser.
Total nitrogen is measured by Kjeldahl digestion of soil (copper sulphate-potassium sulphate
catalyst). Ammonium and nitrate are usually determined in the soil by extracting them by 1M
KCl solution and measured by automated colorimetry (ammonium by salicylate; chlorine and
nitrate by reduction, diazotisation and coupling with N-1-naphthylethylenediamine
dihydrochloride).
Soil nitrogen is usually calculated by commercial laboratories from the organic matter
present. If organic matter is relatively low (e.g. 1-2 per cent) then there may not be enough
natural nitrogen in the soil for adequate crop growth and extra N fertiliser should be applied.
Plants have a higher requirement for nitrogen than for any other nutrient, but the amount that
needs to be applied in fertilisers will depend on the yield targeted and the supply from natural
sources like organic matter breakdown and atmospheric fixation by legumes. If plants have
a good supply of all other nutrients, then growth can be controlled with nitrogen applications.
Only enough nitrogen to support the production target should be applied, because excess
growth may exhaust water supply too early and result in ’haying off’. Too much nitrogen will
produce excess lush (soft) shoot growth at the expense of root growth. Excessive shoot
growth may be more susceptible to disease and insect attack. Too little nitrogen will result in
limited pale shoot growth and the crop or pasture will tend to thin out and be prone to weed
invasion. N is also important in increasing protein level in cereals.
37
UNDERSTANDING SOIL ANALYSIS
For good N utilisation by plants, micronutrients, particularly molybdenum, should be
adequate. Mo is essential for N assimilation in plants because it is a constituent of nitrate
reductase enzymes which are responsible for reduction of NO 3 to NH+4 for subsequent
assimilation into amino acids and protein.
Nitrogen deficiency symptoms in cereals
Symptoms first occur on the oldest leaves. Pale green plants change to yellow, then to
almond white as the deficiency becomes more severe. However, leaves may not die for
some time. N deficiency symptoms are similar to K and P deficiency symptoms, but the
plants die more slowly.
10.
Sulphur (S)
The most important function of S in plants is its involvement in protein synthesis. Sulphur is
present in the structure of the amino acids cysteine and methionine, both of which are
important components of proteins. As much as 90 per cent of the total sulphur in plants may
be present as protein-S.
Sulphur soil testing in Australian soils is treated with caution due to the lack of awareness of
responses to S (Lewis 1999). This may be due to a substantial seasonal variation in plantavailable S when measured with the more common techniques. This variation is caused by
temperature and moisture changes in the soil, which in turn affects the rate of mineralisation
of organic S, as well as S losses due to leaching. Sulphate adsorption capacity or the
measurement of a soil’s ability to hold sulphate against leaching (Barrow 1975), has been
given much priority over the supply of available S when interpreting tests in WA. Fine
textured soils and soils with high organic matter have much greater ability to adsorb
sulphate.
In Western Australian soils S deficiency is usually confined to sandy and sandy loams (Clark
& Lewis 1974). In the Geraldton area where sandplain soils predominate, 35 per cent of
wheat samples were S deficient (Robson et al. 1995). S in the soil is either immobilised or
mineralised, depending on the soil environment. Organic S accumulates in the soil from
stubble, pasture and animal residues and to some extent from reduced tillage, leading to an
increase in the S pool. For example, the accumulation of S in a Coolup sand can reach
4.4 kg/ha/year (Barrow 1969). Soil micro-organisms are primarily responsible for
mineralisation of organic S. Therefore those factors that affect micro-organism populations,
such as temperature, moisture, pH and food supply will determine organic S availability.
Temperature in the range 20-40oC and moisture in the range 15-40 per cent appear to favour
mineralisation. Development of soil testing procedures that characterise net S mineralisation
as well as determining the residual amount of SO42 within the root-zone is needed. A simple
procedure for assessing the SO42 retention in subsoil will assist the prediction of SO42
movement in soil profiles and thus the likely fate of residual SO42 (Anderson et al. 2006).
Brennan and Bolland (2006) have identified responses of canola when soil test values (KCl
procedure) were less than 7 mg/kg in the 0-30 cm soil depth in sandy duplex soils.
Sulphur deficiency symptoms in cereal
Pale green plants have greater yellowing in the newest leaves. Whole leaves are light yellow
with no strips or green veins. Leaf tips of old leaves can die in cases of severe deficiency.
P deficiency may look similar to N deficiency except that in N deficiency only old leaves are
affected first, not the whole plant.
38
UNDERSTANDING SOIL ANALYSIS
11.
Phosphorus (P)
Phosphorus is important for cell division and growth. It is needed for photosynthesis, sugar
and starch formation, in energy transfer and movement of carbohydrates around the plant. It
is concentrated in the growing tips. A deficiency of phosphorus will greatly reduce the growth
of plant tops and roots. It is essential for early root development and plants need a readily
available source of P in the early growth stages, or crop yields will be reduced.
The phosphorus pool in the soil consists of several interacting sources. The soil solution is
the immediate source of P for the plant root, and the rhizosphere is the zone in which the
interaction between the plant and soil solution occurs. Diffusion from bulk soil continuously
replenishes depleted P at the soil. However, because roots take up P in the orthophosphate
(mineral) form, organic P must be mineralised by extracellular micro-organisms (using the
phosphatase enzyme) before uptake can occur.
Increasing Al and Fe oxides in soil increases P adsorption, which in turn makes it less
available to the plant.
The ability of plants to recover P from the soil is dependent on a symbiotic relationship
between the roots of most crops and soil fungi (mycorrhiza), the hyphae of which greatly
increase the surface area for P absorption by roots (Figure 22).
Figure 22. Fungus hyphae greatly increase the surface area for P absorption.
There are three problems with natural P in soil. First, the total level of most soils is usually
low, at about 1000 kg/ha in the upper 15 cm. In WA soils it is much lower, at < 500 kg/ha,
which is no more than one-tenth to one-fourth that of N and one-twentieth that of K (Brady &
Weil 2002). Second, most of the P compounds in the soil are unavailable because they are
highly insoluble. Third, when soluble sources of P are added to the soil as fertilisers and
manures, they are fixed and in time form highly insoluble compounds.
Most laboratories express phosphorus as kg/ha of P rather than as P2O5. This makes it
easier to calculate the amount of fertiliser required to meet an identified deficiency. For
example, if the deficiency is 50 kg/ha P then it will require:
250 kg DAP (20% P) or
500 kg rock phosphate (10% P) or
39
UNDERSTANDING SOIL ANALYSIS
556 kg Superphosphate (9% P).
Although the actual application rate will depend on economic considerations, the soil test will
give a good basis for fertiliser planning. There are different procedures for P test in the soil.
However, researchers in Western Australia have found that they all predict yield equally
(Bolland et al. 1988, Kumar et al. 1994). Accordingly the Colwell (1963) sodium bicarbonate
soil test is commonly used. Some laboratories also use Olsen or Recovery methods.
Colwell method
Bicarbonate P is measured by the method of Colwell (1963), viz. 1:100 soil:solution ratio,
extractant is 0.5 M sodium bicarbonate (pH 8.50), 16 hours extraction at 23qC.
P-Recovery method
P-Recovery is the proportion of P that is available to plants following fertiliser application
after interaction with the soil. For example a P-Recovery of 50 per cent means that if 10 kg
of phosphorus is applied to the soil as fertiliser, only 5 kg would be available to the plants.
P-Recovery can be used to assist determination of the desired level of P for a particular soil.
A high P-Recovery means a lower reading of P is acceptable, but if P-Recovery is low
(< 40%), then a higher level of available P is required.
Phosphorus Retention Index
Phosphorus Retention Index is used to rank the soil's ability to either leach or ’fix‘ phosphate
applied as water-soluble fertiliser. The plant yield response curve for Colwell P (see below)
varies with the soil type, especially influenced by the P adsorption properties. For example,
critical levels (soil test P required for 90 per cent of maximum yield) vary from less than
10 ppm P on grey sands up to greater than 40 ppm P for clay loams. This means that it
requires both a Colwell-P soil test value and a PRI value to come up with a phosphate
fertiliser recommendation (Allen & Jeffery 1990, Allen et al. 2001).
Two points should be considered when dealing with the profitability of P fertilisers:
(i)
The relationship between the amount of freshly applied fertiliser and yield (yield
response curve)
(ii)
The relationship between soil test and yield (Soil P test calibration).
Both the yield response and soil test calibration must be determined locally via field
experiments.
Because plant root systems differ in their physiological P uptake, mycorrhiza dependence
and root surface area, the critical soil test value which is applicable to one crop may not be
applicable to another. A soil test may be correlated with P availability to a crop, but this does
not mean it is also correlated with P fertiliser requirement. The buffering capacity of the soil
for P must be considered before judging the correlation. Thus, for a soil at the same level of
extractable P, the higher the P buffer capacity, the greater the fertiliser requirement.
Broadcasting or dispersing the fertiliser also depends on the buffering capacity of the soil.
Banding fertiliser is much less affected by the buffering capacity while dispersing it is highly
affected by the buffering capacity.
When the P-fixing capacity of a soil is not saturated, a great portion of the applied P will be
fixed by the soil and for optimum crop yield it will take far higher rates of P fertilisers than the
plant actually takes up. It is better in this case to band P fertilisers to reduce the amount
fixed by the soil. In alkaline soils, combining P with ammonium would increase P uptake by
the plant because the nitric acid produced through the oxidation of the ammonium ions slows
40
UNDERSTANDING SOIL ANALYSIS
the formation of the more insoluble calcium phosphate compounds. Addition of organic
matter would also increase the availability of P for the plant. Maintaining soil pHw at about
six to seven would optimise the availability of P in most agricultural systems.
P deficiency symptoms in cereals
Reduced early growth and vigour, dull dark green or purple leaves. Tips begin to yellow and
then the yellow area moves down the leaf. Leaves then die quickly, with the tip becoming
orange to dark brown. P yellowing symptoms of the oldest leaves are similar to N. However,
yellow areas in P die much more quickly.
12.
Calcium (Ca)
Calcium is essential for the proper growth and function of root tips. It is also found in large
quantities in leaves where it is a constituent of the cell wall. Calcium is thought to encourage
earthworm activity and thus can increase the natural aeration of the soil. A good level of
calcium is also associated with aggregation and stable soil structure. This is due to an
increase in flocculation of the clay particles and to less extent, to the increased activity of soil
organisms. Both an excess and a deficiency of calcium will create a phosphate deficiency.
Factors contributing to Ca deficiency may include alkaline sodic soils and high soluble Al.
Absolute deficiency of calcium for plants is not common in Australian soils. However, many
Australian soils show inadequate concentration for healthy soil structure. Acid soils with low
CEC in high rainfall environments are most likely to be low in Ca.
Ca is the fifth most plentiful element and the third most plentiful metal (after Fe and Al) in the
earth’s crust. Its content in the lithosphere (earth's crust and upper mantle) is about 3.6 per
cent. However, Ca in non-calcareous soils is much lower ranging from about 0.1 to about 1
per cent. Ca is lost through crop removal, leaching and soil erosion, but is replenished by
weathering and dissolution, by decay of organic matter, and by addition of fertilisers and
amendments such as lime, dolomite and gypsum. Under humid conditions, particularly with
highly weathered soils, removal exceeds replenishment and base unsaturation of the
exchange complex develops. Under such conditions soil becomes strongly acid and Ca
deficiency may develop (Bruce 1999). The level of exchangeable Ca reflects the interacting
effects of the total amount and the solubility of Ca sources, CEC, competition from Al, Mg or
Na, and the extent of the Ca-removing processes. In general, neutral and alkaline soils
possess higher concentrations of exchangeable Ca relative to acid soils.
Calcium saturation is the percentage of the CEC occupied by exchangeable Ca and can vary
widely (Table 13). Ca saturation is correlated with soil pH and inversely related to Al
saturation. Ca saturation is very important for soil structure. Low Ca saturation commonly
results in deteriorated soil structure. In this case soil amendment such as gypsum or lime is
required to increase the saturation percentage.
Exchangeable Ca level commonly exceeds that of exchangeable Mg level, except in humid
and sub-humid regions where exchangeable Mg increases relative to exchangeable Ca as
soil age and degree of development increases (Buol et al. 1973).
Since exchangeable Ca is the principal source of soluble Ca, the concentration of Ca in soil
solution is influenced by the amount of exchangeable Ca and also by factors which affect its
release from exchange sites. These include the nature of the exchange complex, the degree
of Ca saturation, and the type and ratio of complementary ions (mainly Mg, K and Na). Ca,
like almost all other nutrients, undergoes temporal changes where the concentration is
reduced by crop removal but restored by the next season. Ca concentration also varies with
soil moisture and seasonal conditions.
41
UNDERSTANDING SOIL ANALYSIS
Table 13. Analytical data for topsoik and subsoil of acid soils (Bruce et al. 1989)
Soil (91 sites)
Topsoil
Subsoil
pHw
5.29
5.21
pHCa
4.47
4.37
Exch. Ca, cmol/kg
2.4
0.74
Ca saturation, %
41
20
Ca concentration, PM
308
790
Ca activity ratio
0.107
0.054
Soil solution (48 sites)
Calcium in the plant
The nutritional effects of Ca can be explained by the following aspects of the physiology:
x
Plants do not have a highly efficient uptake mechanism for Ca
x
Ca uptake by the root occurs just behind the tip
x
The upward movement of Ca from the roots is controlled by the rate of transpiration
x
Redistribution of Ca from old to young leaves is negligible because very little Ca is
remobilised or translocated downward in the plant as the concentration of Ca in the
phloem is very low.
Calcium can be supplied as agricultural gypsum, lime or dolomite and also in calcium and
rock phosphate fertilisers. Dolomite is used where there is also a magnesium requirement,
or where gypsum application will create a soil magnesium deficiency.
Soil tests for Ca
Measurement of exchangeable and soil solution Ca are the bases for diagnostic soil tests for
Ca. Calcium level may be used directly or expressed relative to other soil attributes. For
example, exchangeable Ca can be expressed as a percentage of the CEC (Ca saturation) or
as the ratio of Ca to Mg. Soil solution Ca is often expressed as the Ca activity ratio, which is
the ratio of Ca activity to the sum of the activities of all major cations in solution.
Calcium/pH
Calcium recommendation is usually based on soil deficiency, not on pH. This
recommendation will be for calcium carbonate (lime), dolomite or gypsum, depending on the
percentages of calcium and magnesium present in the soil. Note that applied calcium will
displace soil magnesium, so dolomite will often be recommended if exchangeable soil
magnesium is low (usually below 1 per cent base saturation). Calcium will also suppress the
uptake of manganese, so a deficiency may be induced by the application of gypsum,
especially if soil manganese is low. However, there is no unique or universal Ca:Mg ratio
applied to all crops and/or soils that gives the highest yield, because different soils and
different crops show different Ca:Mg requirements for highest yield.
Calcium deficiency symptoms in cereals
Spots in the middle of the newest leaf die. This area quickly expands and the leaf collapses
in the middle before it even unrolls. Terminal growth is distorted. Deficiency symptoms are
seldom observed in Western Australia.
42
UNDERSTANDING SOIL ANALYSIS
13.
Magnesium (Mg)
Magnesium is essential for both animal and plant production. Magnesium is the eighth most
common element in the earth’s crust (Barber 1995). The relative abundance in plant matter
is similar to that of S and P but less than that of N, K and Ca. The bulk of Mg in the soil is
usually in a form of primary and secondary minerals that is not readily available to the plant.
Despite an Australia-wide recognition of the soil acidity problem, with an associated
implication of declining soil Mg levels, there is little documentation in Australia of the effect of
applied Mg on yield or the relationship between soil test values and yield response to applied
Mg (Aitken & Scott 1999). For this reason, Mg is only applied occasionally as a fertiliser.
Exchangeable Mg has proved to be a good estimate of total plant-available Mg and soil tests
based on this are likely to be successful in identifying the soil Mg status. Soil tests are
usually reliable because Mg in soil changes little over time. The major Mg inputs to the soil
are from the breakdown of certain minerals, rain and fertilisers, while the major losses
include crop removal and leaching. Both inputs and losses are relatively small.
The desired level of base saturation percentage for Mg is around 10-20 per cent (which
depends on the soil CEC).
Magnesium deficiency symptoms in cereals
Leaves are pale and soon yellow, remaining unopened with a twisted appearance. With time
yellowing becomes mottled (spots/beads) and in old leaves may turn reddish along the leaf
margins. In severe deficiency the entire length of the leaf will remain folded or rolled.
14.
Potassium (K)
As an essential macronutrient, K is required for photosynthesis, translocation, cellulose
formation, enzyme activities, cation-anion balance and stomatal control (Marschner 1986).
Crops require a large quantity of K, which has to be supplied from the soil. Potassium is one
of the most abundant elements in the soil. Soil K is a function of the parent material, the
extent of weathering and leaching of soil minerals, the type of soil minerals, organic matter
content and K fertilisers. The total K content of soil generally increases with increasing clay
content, giving texture an important role influencing both total and exchangeable K
(Table 14).
Potassium deficiency in Australian agricultural soils has been long recognised (Gourley
1999), particularly in the south-west of WA (Williams & Raupach 1983). A reliable soil test is
based on ’exchangeable K‘ or ’extractable K’. There is little evidence that one test is better
than another.
Table 14.
Total and exchangeable K in some Western Australian soils (after Cox 1973)
Location
Texture
Total K, %
Exchangeable K, mg/kg
Busselton
Coarse sand
0.02
16
Walpole
Fine sand
0.06
14
Cundinup
Loam
1.8
120
Bridgetown
Clay
1.4
160
Most of the widely used soil tests have had some level of field calibration which has resulted
in suggested critical values that form the basis of K fertiliser decisions for many crops and
43
UNDERSTANDING SOIL ANALYSIS
pastures. Surprisingly, these critical values in surface soils are similar for most
agronomically important plant species and are generally around 0.2-0.5 cmol(+)/kg or
80-200 ppm for Australia in general, and around 60 ppm for mostly duplex soils in WA
(Gourley 1999). The desired level of K depends on the exchange capacity (CEC) of the soil.
Light soils (less than six CEC) require a higher percentage of K (6 per cent). Heavier soils
(greater than 20 CEC) require 3 per cent K. This is due to the fact that there are more
exchange surfaces in a high CEC soil so there is also a higher amount of potassium
available. Plants require relatively large amounts of potassium.
Furthermore, high Ca and Mg levels in the soil solution may reduce K uptake by the plant
roots. Due to the competition between K and Ca, K deficiency frequently occurs in
calcareous soils even when the amount of exchangeable K present would be adequate for
plant nutrition (Brady & Weil 2002).
The desired level of K in the soil is around 2 to 5 per cent base saturation.
Potassium deficiency symptoms in cereals
Symptoms first appear on the oldest leaves, speckled along their whole length, quickly
spreading to the tip and margins in severe cases. As leave die back from the tip and
margins a spear shaped pattern of green remains
15.
Cobalt (Co)
Co is an essential trace element for animals, but not for plants except legumes, where it is
required by rhizobia in legume nodules for N fixation (Peverill & Judson 1999). Cobalt
deficiency in legumes is shown by reduced and undeveloped nodules.
Soluble or available Co is determined by various extractants including 2.5 per cent acetic
acid (pH 2.5), neutral N ammonium acetate, or 0.05 M EDTA (Mengel & Kirkby 1982).
Co absorbed through the leaves is particularly immobile, while that absorbed by the roots
follows the transpiration stream and is concentrated at the margins and tips. Excess Co may
induce Fe deficiency.
Co is more likely to be deficient during wet periods and periods of rapid, fresh growth
because the Co taken up is distributed through a larger plant mass. Deficiency occurs in
highly leached sandy soils derived from acid igneous rocks or in highly calcareous or peaty
soils. Co deficiency can be controlled by applying cobalt salt, usually sulphate, to the soil.
In Western Australia, the critical level for Co has not been determined yet.
16.
Boron (B)
Boron is essential for photosynthesis and energy production in plants. It also acts as a
modifier in helping to maintain the balance between calcium, potassium and magnesium,
especially when one of these major cations is in excess and creating a major imbalance. It is
needed for the development and growth of new cells, and since it is not readily translocated
within the plant, boron increases calcium absorption in plants and animals, increases fruit
quality and shelf life and also increases the oil percentage in canola. Boron is also involved
in many other essential plant processes, including the translocation of sugars and other biochemicals, protein synthesis, nodule formation in legumes and the regulation of carbohydrate
metabolism (energy). In pastures, B helps to balance nitrogen levels and prevents the
excess accumulation of nitrate nitrogen, thus reducing excess protein in spring and autumn
pasture. This is also the case in fruit crops.
44
UNDERSTANDING SOIL ANALYSIS
The absorption of B by plant roots is determined by its concentration in the soil solution and it
is correlated well with assessing toxicity and plant response on a wide range of soils (Aitken
& McCallum 1988).
Soil testing is difficult because of high spatial variability and the high B concentrations that
occur in the subsoil which is more difficult to sample. Boron toxic patches scattered in the
field occur irregularly, impairing growth and causing yields to be low. Soil samples can be
collected from such patches for analysis.
In Australia three methods are in use in testing for B:
x
Hot water soluble extraction method (Haddad & Kaldor 1982) which does not work well
in sodic or alkaline clay soils
x
CaCl2-Mannitol extraction method (Cartwright et al. 1983)
x
Hot CaCl2 (Spouncer et al. 1992).
The CaCl2 is mainly used to assess B toxicity on alkaline and sodic soils for which the hot
water soluble B test is unsatisfactory.
In WA, B toxicity rather than B deficiency is a widespread constraint to cereal and legume
crop and pasture production in alkaline soils. However B deficiency has been reported on
some sandy soils (Bell 1999). The B absorption rate by plant roots is determined by
concentration in the soil solution. For assessing B toxicity, soil solution B appears to
correlate well with plant response on a wide range of soils (Aitken & McCallum 1988).
Generally, soil analysis gives a reasonable prediction of B deficiency when tests are
calibrated for particular soil groups and crop species. Both hot water and hot 0.01M CaCl2
extractant are used for soil B analysis in Australia. In soils with low B, the two extractants
remove similar amounts of B and correlate well with plant response. Most values of the
critical concentration for deficiency range from 0.15 to 0.5 mg B/kg soil. In very sensitive
crops and in alkaline clay soils these values can be doubled (Bell 1999). The capacity of soil
to buffer soil solution B against losses by plant uptake is determined by soil texture, soil
mineralogy, pH and organic matter. B reacts more strongly with clay than with sandy soils,
making clay soils buffer B in the soil solution better than sandy soils. B is also less likely to
leach through clay soils or clay-rich horizons than sandy soils or horizons (Pinyerd et al.
1984). Higher organic matter also increases the B-sorption capacity of soil (Yermiyahu et al.
1995).
Boron deficiency symptoms in cereals
The first visual sign of boron deficiency is often the cessation of growth of buds or shoots.
This is followed by the death of young leaves, so deficiency often gives plants a bushy
appearance. Wheat heads can also have a bushy appearance and show white tips. Newer
leaves split along the leaf close to the midrib and a saw tooth effect on edges of young
leaves may be seen. Shoots wither similarly to Cu and Ca deficiencies.
Boron toxicity
In contrast to all other essential plant nutrients, B is mainly present in the soil solution as non
ionised boric acid, B(OH)3 which under high soil pH conditions is only slightly de-protonated
(dissociated). This is the main reason why B is easily leached from the soil. However, in
compacted, poorly drained soils or soils with a subsoil hard pan, B can accumulate to a toxic
level. Boron toxicity symptoms include the yellowing and death of leaf tips, starting with the
oldest leaves first. These symptoms often do not appear in early vegetative growth.
45
UNDERSTANDING SOIL ANALYSIS
17.
Iron (Fe)
Iron has many important functions in plants such as involvement in the synthesis of
chlorophyll.
Iron is the fourth most abundant element in the earth’s lithosphere after oxygen, silicon and
aluminium. Many extraction methods have been developed for use in soil taxonomy
systems, but they are inappropriate for the assessment in nutrient availability because they
extract not only the most reactive forms of Fe, but also varying amounts of less reactive
forms of little use to plants. It is difficult to recommend a particular soil analysis for predicting
deficiency in plants. In Fe deficiency, soil environment factors such as pH, moisture content,
temperature and bicarbonate concentration and the genetic variability within and between
plant species with respect to Fe uptake, make any Fe soil-testing procedure very difficult to
calibrate for reliable interpretation (McFarlane 1999). Fe solubility is the most important
factor responsible for Fe deficiency in plants, especially on calcareous soils. Fe solubility
decreases significantly in aerated soils especially at soil pHw 7-8, where most of the Fe is
present as low soluble Fe3+ oxides and hydroxides. Soil pH is the most important soil
parameter influencing Fe solubility and hence availability to plants. The concentration of Fe
in soil solution within common soil pH levels ranges from 30 to 550 g/L, whereas in strongly
acid soil it can exceed 2000 g/L. When a soil is submerged, Fe3+ compounds are directly
and indirectly reduced to the more soluble (more available) Fe2+, and large amounts of Fe
are brought into solution. The concentration of water-soluble Fe2+ increases to a peak value
and then declines or reaches a plateau. These changes depend on the pH and organic
matter content, temperature and the salt content of the soil. However poor aeration,
especially in compacted soils which lead to higher solubility of Fe, may intensify Fe
deficiency because many smaller roots are injured or destroyed, which in turn reduces the
absorptive capacity of the whole root system.
The content of soluble Fe in soil is extremely low in comparison with the total content. Most
available Fe is the reduced ferrous Fe2+ which decreases availability in well aerated soil.
Furthermore, Fe in general and Fe3+ in particular, are highly soil pH dependent, where the
activity of Fe3+ falls with increasing pH. At higher pH level Fe3+ activity in solution decreases
1000 fold for each pH unit rise and reaches its minimum at 6.5-8.0. Since Fe is more soluble
in acid soils, Fe deficiencies can be caused by over-liming.
The critical Fe level in WA has not been determined but can range from 100 to 400 ppm.
Iron deficiency symptoms in cereals
Pale green to yellow leaves. Interveinal chlorosis (usually yellow colour turning white under
severe deficiency). Similar symptoms to N and P deficiencies, usually seen in youngest
leaves.
18.
Manganese (Mn)
Manganese is essential for many important plant functions, including photosynthesis,
nitrogen metabolism and nitrogen assimilation. Manganese is essential for calcium uptake
by plants. Deficiency symptoms are common in cereals growing on highly alkaline soils and
generally appear as a yellowing of the leaves. Roots are covered by a thick film which is
easily wiped off, leaving a thin white strand. There are pale green strips on young leaves,
which are weak and tear easily.
It is almost impossible on the basis of soil test alone to correctly diagnose either Mn
deficiency or toxicity of field-grown plants. Although much is known of the nature and
reaction of Mn in soil and of its absorption and functioning in plants, our ability to predict the
46
UNDERSTANDING SOIL ANALYSIS
level of plant-available Mn in soil is limited (Uren 1999). Determination of the Mn status of
field soils for adequate plant growth requires measurement of water-soluble plus
exchangeable Mn, easily reducible Mn, soil pH and Mn concentration in the plant. Time of
sampling, field conditions at the time of sampling, time taken for and conditions of drying and
storage before analysis can all affect test values. Standard protocols for sampling,
preparation and storage need to be established and adhered to strictly in routine soil analysis
(Uren 1999).
Mn deficiency was first reported in WA by Carne in 1927. Toxicity is not as widespread as
deficiency. The availability of Mn in soil is determined by soil and plant factors. Soil factors
include microbial activity, pH and the reducibility of Mn oxides. The reducibility of the oxides
is related to the exposed surface area of these oxides. Amorphous oxides with small particle
size (large specific surface area) are the most reactive. Plant factors are related to the
surface area and reducing capacity of the roots. Quite large differences between species
and cultivars of the same species exist in their tolerance of toxicity and deficiency. However
there is no apparent link between the mechanism of tolerance to toxicity on one hand and
tolerance to deficiency on the other.
Mn concentration in plants can also be suppressed in high potassium soils or following
calcium or potassium applications.
The desired Mn level (total) in soil is between 80 to 140 ppm.
Mn toxicity
Gray flakes or yellow or brown spots develop on leaves. Note that the flecking symptoms
are similar to insect damage, while the yellow spots are similar to some leaf diseases or
herbicides. Brown spots in barley can be confused with leaf disease.
19.
Copper (Cu)
Copper is involved in many plant processes. It is essential for photosynthesis and for protein
and carbohydrate metabolism. Copper also appears to be required for nitrogen fixation by
Rhizobia. In severe deficiencies leaf tips turn almost white and leaves become narrow. In
cereal grains the seeds fail to develop leaving blind ears. Yellow bleached spots on the
surface of leaves may occur, similar to the symptoms of Mn toxicity. Copper is an essential
nutrient for animals and is often deficient when they graze pastures low in copper.
Many empirical methods have been used to test for Cu in soil. We call them empirical
because there is no guarantee that the Cu extracted in soil is necessarily the one that will be
absorbed by the roots. Most soil tests extract much more Cu than is accumulated by the
shoots and roots of the annual crops. The most reliable method in showing the amount of
Cu that can be available to the plant is when Cu is extracted with ammonium oxalate. Using
this method in WA, it has been shown, for example, that 0.3 ppm Cu or less in the topsoil
(top 10 centimetres) is certainly indicating a deficient Cu level for wheat, while greater than
0.8 ppm is almost adequate.
Deficiency level of Cu, Zn, Mn and Fe can be identified by the DTPA procedure, but this test
has not been calibrated adequately for acidic WA soils.
Copper deficiency of cereals in Western Australia was first recognised in late 1930s and
early 1940s (Teakle 1942). Researchers have reported widespread Cu deficiency on the red
gum country, sandy, gravelly soils of the wheatbelt, Great Southern soils and coastal soils
high in lime (Teakle 1942, Dunne 1948). Soil factors causing deficiency or toxicity include
soil pH, organic matter, clay, iron and aluminium oxides as well as soil moisture and
47
UNDERSTANDING SOIL ANALYSIS
temperature. It has been reported that organic matter and Fe and Al oxides in WA soils
strongly bind Cu and influence its availability to plants (Brennan 1986).
Copper deficiency can be induced by excess soil molybdenum, and iron and copper
availability to plants is affected by the soil pH. As the pH increases, so copper availability
decreases.
The desired level of Cu in soil extracted by ammonium oxalate is greater than 0.8 ppm. The
desired level in soil extracted by DTPA is around 0.2-0.3 ppm (Ross Brennan, DAFWA, pers.
comm.).
20.
Zinc (Zn)
Zinc is an essential nutrient for both animals and plants. In plants it promotes growth
hormones and starch formation and is involved in seed maturation and production. Grain
seed heads often fail to completely fill if zinc is deficient. It is essential for root membrane
permeability and moisture absorption into roots.
There is no single reliable soil test for Zn in Australia (Armour & Brennan 1999). DTPA is the
most widely used test particularly in recent research. Calibration data are only available for a
limited number of plant species and soil types. Plant species and selections within species
vary widely in their tolerance to Zn deficiency as well as toxicity (Graham & Rengel 1993).
The most important soil factor associated with Zn deficiency is pH, because availability is
markedly reduced with increasing soil pH over the agriculturally important range of 5.5-7.0.
For this reason over-liming acid soils may severely reduce the Zn availability for the plant.
Highly available P and prolonged waterlogging also reduce availability. Zn is present in soil
in different forms, including in soil solution as Zn2+, ZnCl+, and ZnOH+. These ions are
immediately available for plants as exchangeable Zn, in complex with organic matter,
occluded by or co-precipitated with oxides and hydroxides of iron, aluminium and
manganese held in clay minerals.
DTPA extraction is widely used in testing Zn in soil; however, wide variation exists between
soils due to different clay content, pH and organic matter. The desired level for Zn in soil
extracted by DTPA is around 0.2 to 0.6 ppm depending on the soil type (Armour & Brennan
1999).
Zn deficiency symptoms in cereals
Young to middle leaves develop yellow patches between the mid-vein and the edge of the
leaf and extend lengthways towards the tip and base of the leaf. These areas eventually die,
turning pale grey or brown. Plants take on a water- or diesel-soaked appearance.
Symptoms are similar to leaf spot.
21.
Molybdenum (Mo)
Molybdenum is essential for both plants and animals. In plants, Mo is part of enzymes
needed for nitrogen fixation by clovers and lucerne, as well being essential for nitrogen
utilisation, especially in brassicas. Deficiencies occur mostly at a pH <5. In livestock, excess
levels in pasture can adversely effect stock health by reducing the absorption of copper and
so induce copper deficiency. This problem has been caused by the excessive use of
molybdenum fertilisers.
Molybdenum was first established as an essential element for higher plants by Arnon and
Stout (1939). Acid ammonium oxalate extraction of soil Mo has received the most attention
48
UNDERSTANDING SOIL ANALYSIS
of all tests. In soil, Mo occurs in clay minerals, bound to organic matter and in soil solution.
The soil solution Mo is readily available for the plant, followed by MoO42 held weakly on the
surface of clay or secondary minerals, while Mo held in primary mineral structure is only
sparingly soluble and considered unavailable to the plant.
At the time of sampling, tissue tests may be able to diagnose Mo deficiency in some crops.
There is no current reliable soil test for Mo.
Mo deficiency symptoms in cereals
Mo deficiency symptoms are difficult to detect in the field, however middle leaves show
speckled flecking or yellow stripes and appear limp and water stressed. At high N levels the
old leaf tips are scorched. Under severe deficiency maturity is delayed and heads are
empty. Cu deficiency may look similar to Mo deficiency. Physiological effects such as
crimping of the flag leaf, are often confused as a symptom of Mo deficiency.
22.
Sodium (Na)
Excess sodium causes dispersion and destruction of soil structure. Dispersed clay particles
block soil pores resulting in restricted water and air movement. If surface soil pores are
blocked by dispersed clay, the soil becomes massive and when dry may form a surface crust
or hard-setting layer. This will inhibit seedling emergence and water penetration. Subsoil
dispersion results in reduced water movement down the profile. Following heavy rainfall,
waterlogging of the surface soil could also result. The consequent lack of oxygen in the root
zone will detrimentally affect the crop. During hot weather, there may also be excessive
evaporation due to the restriction of drainage through the profile. On sloping land, lateral
water movement may occur above the relatively impermeable subsoil horizon. Subsoil
dispersion also results in the structural breakdown of aggregates and increased soil density
and hardness. Plant root movement will be restricted by the dense subsoil and much of the
moisture and nutrients stored in the subsoil will not be used.
In Australian soils, when the exchangeable Na percentage (ESP), exceeds 5 per cent of the
cation exchange capacity, soil structure starts to degrade especially in soils with low Ca.
This degradation of structure significantly reduces water infiltration and percolation. Gypsum
is usually applied to address soil sodicity. However, lack of rain and drainage may impose
serious problems in improving sodic soils because there is not enough water to help calcium
replace sodium and there is no outlet for the leached sodium. Sodic topsoils can also benefit
from addition of organic matter, because organic matter increases cation exchange capacity
and helps bind soil particles together.
Acceptable Na level in soil is between 0.5 to 2 per cent of the CEC.
Note: Often a preferred index or measure of sodicity is the Sodium Adsorption Ratio (SAR)
rather than Exchangeable Sodium Percentage (ESP), because ESP values can vary
depending on whether CEC or the sum of the major cations is used as the divisor. SAR is
often reported in horticultural analyses.
49
UNDERSTANDING SOIL ANALYSIS
23.
Aluminium (Al)
Aluminium is not an essential nutrient for most higher plants, but can still be taken up and
accumulated through passive transports such as osmosis or with the flow of transpiration
water, possibly through damaged membranes. The importance of Al in soil stems from its
abundance where 15 per cent of the earth’s crust is made of Al2O3, thus making Al an
important constituent of the soil. Furthermore, Al and Si, are the major elements making up
the lattices of soil clay minerals. The solubility of Al increases sharply when soil pH
decreases below 5.5 and more than half the cation exchange sites may be occupied by Al.
The first observable effect of Al toxicity on plants is a limitation in root growth. Root tips and
lateral roots become thickened and turn brown. Frequently P uptake and translocation to the
upper parts of the plant are decreased. There is a wide variation in Al tolerance between
species. Lucerne, barley, medics and canola are highly sensitive, while oats, triticale and
lupins are very tolerant. Aluminium is only a problem in low calcium soils, so addressing the
lime or dolomite requirements is the priority and will usually overcome toxicity challenges.
Soil aluminium levels reported from soil analysis are generally not a reliable indicator of lime
requirement and it is better to apply lime based on the level of exchangeable calcium
present.
Acceptable 0.01 M CaCl2 Al level in soil is less than 4 ppm, frequently depending on the
sensitivity of the plant species (Slattery et al. 1999).
Symptoms of Al toxicity
Poor lateral root development and the main root is thickened and distorted in taprooted
species. Early in the season all leaves become pale, particularly the oldest, which turn
yellow and die. Leaf symptoms are similar to P deficiency. Late in the season plants appear
to be drought stressed despite adequate soil moisture. However, tolerant crops do not
display clear symptoms.
50
UNDERSTANDING SOIL ANALYSIS
Glossary of soil science terms
Acid cations: Cations principally Al3+, Fe3+, and H+, that contribute to H+ ion activities either
directly or through hydrolysis reaction with water.
Acid soil: Soil with a pH value <7.0.
Active acidity: The activity of hydrogen ions in the aqueous phase of a soil. It is measured
and expressed as pH value.
Adsorption: The process by which atoms, molecules, or ions are taken up from the soil
solution or soil atmosphere and retained on the surface of a solid by chemical or physical
binding.
Aggregates: A group of primary particles that cohere to each other more strongly than to
other surrounding particles, such as a clod, crumbs, block, or prism.
Agronomy: The theory and practice of crop production and soil management.
Alkaline soil: Soil with a pH value >7.0.
Antagonism: Production of a substance by one organism that inhibits one or more other
organisms. The terms antibioses and allelopathy have also been used to describe such
cases of chemical inhibition.
Available nutrient: (i) The amount of soil nutrient in chemical forms accessible to plant
roots or compounds likely to be convertible to such forms during the growing season; and
(ii) The contents of legally designated ’available‘ nutrients in fertilisers determined by specific
laboratory procedures which in most states constitute the legal basis for guarantees.
Base saturation percentage: The extent to which the adsorption complex of a soil is
saturated with exchangeable cations other than hydrogen and aluminium. It is expressed as
a percentage of the total cation exchange capacity
Bulk density: The mass of dry soil per unit volume of soil with units of g/cm3, mg/m3 or t/m3.
The bulk volume is determined before drying to constant weight at 105°C.
Cation Exchange Capacity, CEC: The sum of exchangeable bases plus total soil acidity at
a specified pH value (usually 7 or 8). The unit is centimoles of charge per kilogram of
exchanger (cmolc/kg).
Colloid: A particle, which may be a molecular aggregate, with a diameter of 0.1 to
0.001 Pm. Soil clays and soil organic matter are often called soil colloids because they have
particle sizes that are within, or approach colloid dimensions.
Dolomite: A mineral consisting of CaCO3 (Ca = 21.7) and MgCO3 (Mg = 13.2) used to
supply Ca and Mg ions.
Essential elements: Any plant nutrient that is required for the completion of the life cycle.
Exchangeable sodium percentage: The extent to which the adsorption complex of a soil is
occupied by sodium. It expressed as follows: ESP = [Exchangeable sodium (cmolc/kg
soil)/total cation exchange capacity (cmolc/kg soil)] X 100.
51
UNDERSTANDING SOIL ANALYSIS
Green manure: Green plant material incorporated with the soil by cultivation, chemical
desiccation or mulching for improving soil organic matter.
Gypsum: Calcium sulphate, CaSO4.2H2O, is used to improve compacted soils and increase
soil permeability and help soil flocculation, which is a prelude for soil aggregations. It is also
used to treat sodic soils where Ca ions can replace Na ions. It has a moderate solubility of
2.41 g/L in cold water and 2.22 g/L in hot water. Similarly, gypsum is commonly used as a
sulphur fertiliser at rates much lower than that required for soil structure improvements.
Kaolinite: An aluminosilicate mineral of the 1:1 crystal lattice group consisting of single
silicon tetrahedral sheets alternating with single aluminium octahedral sheets. A very
common clay mineral in Western Australia.
Lime: CaCO3, mainly used to remove acidity (H+) from the soil by converting the hydrogen
ions to water and releasing CO2 gas. It has low solubility of 0.014 g/L (cold water) and
0.018 g/L (hot water).
Macronutrients: Essential plant element needed by the plant in relatively large quantities
such as N, P, K, Ca, Mg, S and Cl.
Micronutrients: Essential plant elements needed in small quantities such as Fe, Mn, B, Cu,
Zn, Co, and Mn.
Mineral soil particles: Minerals particles <2.0 mm in equivalent diameter ranging between
specified size limits. The International Society of Soil Science recognises the following soil
separates: coarse sand 2.0 to 0.2 mm; fine sand 0.2 to 0.02 mm; silt 0.02 to 0.002 mm and
clay <0.002 mm.
Mineralisation: The conversion of an element (for example, N and S), bound in an organic
form, into an inorganic state as a result of microbial decomposition.
Montmorillonite: An aluminosilicate clay mineral in the smectite group with a 2:1 expanding
crystal lattice, with two silicon tetrahedral sheets enclosing an aluminium octahedral sheet.
Isomorphous substitution of magnesium for some of the aluminium has occurred in the
octahedral sheet. Considerable expansion may be caused by water moving between silica
sheets of contiguous layers.
Muriate of potash: A potassium fertiliser, potassium chloride, containing 49 per cent of
potassium element.
Oxidation: The loss of one or more electrons by an ion or molecule.
Ped: A unit of soil structure such as a block, column, granule, plate, or prism, formed by
natural processes (in contrast with a clod, which is formed artificially.
Pedon: A three dimensional body of soil with lateral dimensions large enough to permit the
study of horizon shapes and relations.
Permeability, soil: The penetration of water and gases through a bulk mass of soil or a
layer of soil. Since different soil horizons vary in permeability, the particular horizon under
question should be designated.
Pore space: The portion of soil bulk volume occupied by soil pores.
52
UNDERSTANDING SOIL ANALYSIS
Potential acidity: The acidity that could potentially be formed if reduced sulphur
compounds in a potential acid sulphate soil were to become oxidised.
Protonation: Attachment of proton (H+) to exposed OH group on the surface of soil
particles, resulting in an overall positive charge on the surface.
Reduction: The gain of one or more electrons by an ion or molecule.
Saline soil: A non-sodic soil containing sufficient soluble salt to adversely affect the growth
of most crop plant. The lower limit of saturation extract electrical conductivity of such soils is
conventionally set at 4 dS/m (at 25°C). Sensitive plants may be affected at half this salinity
while highly tolerant ones may be affected at about twice this level.
Soil erosion: The detachment and movement of the soil surface by water, wind, ice or other
geological agents including gravitational creep.
Soil horizon: A layer of soil or soil material approximately parallel to the land surface and
differing from adjacent genetically related layers in physical, chemical, and biological
properties or characteristics such as colour, structure, texture, consistency, kinds and
number of organisms present, degree of acidity or alkalinity, etc.
Soil strength (cone index): It is a transient localised soil property which is a combined
measure of a given pedon or other subunit’s solid phase adhesive as well as cohesive
status. It is highly influenced by soil moisture and bulk density. Other soil factors such as
organic matter and soil texture may also affect it.
Soil structure: The arrangement or combination of primary soil particles into secondary
units or peds. These secondary units are characterised on the basis of size, shape and
grade.
Soil test interpretation: Using soil data to improve or mitigate a problem(s) in the soil and
that leads to increase net yield and profit. Some interpretations may take environment into
account.
Soil texture: It refers to the relative proportion of the soil primary particles of different size
ranges - sand (S), loam (L) and clay (C).
Subsoil: It is located just beneath topsoil and extended from 10 centimetres to include
almost all soil rooting depths. It is usually low in organic matter and high in leached
nutrients.
Topsoil: The surface soil layer that is about zero to 10 centimetres in depth and separates
the subsoil from the atmosphere. It usually has a darker colour because soil organic matter
is highest.
Total acidity: Approximated by the sum of the salt-replaceable acidity plus the residual
acidity.
53
UNDERSTANDING SOIL ANALYSIS
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